Don't emit source mappings for removed strings.

[kzc]

Fixes #149

Also fixes two incorrect test results and the rollup issue:
rollup/rollup#3001

[kzc]

@Rich-Harris Please take a look. If the approach in this PR is incorrect you can use the following to verify your results for a different fix:

Here are the dumps of the original incorrect test source maps using source-map@0.6.1:

$ cat dump-original-test-sourcemaps.js 
[
    '{"version":3,"file":"output.md","sources":["input.md"],"sourcesContent":["abcdefghijkl"],"names":[],"mappings":"AAAA,CAAC,CAAC,CAAC,AAAM,CAAC,CAAC"}',
    '{"version":3,"file":"output.md","sources":["input.md"],"sourcesContent":["abcdefghijkl"],"names":[],"mappings":"AAAA,GAAG,GAAG,AAAG,CAAC"}',
].forEach(function(json) {
    console.log(json);
    var sourcemap = JSON.parse(json);
    var consumer = new require("source-map").SourceMapConsumer(sourcemap);
    consumer.eachMapping(m => console.log(JSON.stringify(m)));
    console.log("-----------------------");
});

$ cat dump-original-test-sourcemaps.js | node
{"version":3,"file":"output.md","sources":["input.md"],"sourcesContent":["abcdefghijkl"],"names":[],"mappings":"AAAA,CAAC,CAAC,CAAC,AAAM,CAAC,CAAC"}
{"source":"input.md","generatedLine":1,"generatedColumn":0,"originalLine":1,"originalColumn":0,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":1,"originalLine":1,"originalColumn":1,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":2,"originalLine":1,"originalColumn":2,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":3,"originalLine":1,"originalColumn":3,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":3,"originalLine":1,"originalColumn":9,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":4,"originalLine":1,"originalColumn":10,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":5,"originalLine":1,"originalColumn":11,"name":null}
-----------------------
{"version":3,"file":"output.md","sources":["input.md"],"sourcesContent":["abcdefghijkl"],"names":[],"mappings":"AAAA,GAAG,GAAG,AAAG,CAAC"}
{"source":"input.md","generatedLine":1,"generatedColumn":0,"originalLine":1,"originalColumn":0,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":3,"originalLine":1,"originalColumn":3,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":6,"originalLine":1,"originalColumn":6,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":6,"originalLine":1,"originalColumn":9,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":7,"originalLine":1,"originalColumn":10,"name":null}
-----------------------

Notice the collisions of the generated line/column combinations for the respective tests above:

{"source":"input.md","generatedLine":1,"generatedColumn":3,"originalLine":1,"originalColumn":3,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":3,"originalLine":1,"originalColumn":9,"name":null}

{"source":"input.md","generatedLine":1,"generatedColumn":6,"originalLine":1,"originalColumn":6,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":6,"originalLine":1,"originalColumn":9,"name":null}

Here are the dumps of the altered source map tests in this PR without the collisions and without the mappings for the dropped characters:

$ cat dump-new-test-sourcemaps.js 
[
    '{"version":3,"file":"output.md","sources":["input.md"],"sourcesContent":["abcdefghijkl"],"names":[],"mappings":"AAAA,CAAC,CAAC,CAAO,CAAC,CAAC"}',
    '{"version":3,"file":"output.md","sources":["input.md"],"sourcesContent":["abcdefghijkl"],"names":[],"mappings":"AAAA,GAAG,GAAM,CAAC"}',
].forEach(function(json) {
    console.log(json);
    var sourcemap = JSON.parse(json);
    var consumer = new require("source-map").SourceMapConsumer(sourcemap);
    consumer.eachMapping(m => console.log(JSON.stringify(m)));
    console.log("-----------------------");
});

$ cat dump-new-test-sourcemaps.js | node
{"version":3,"file":"output.md","sources":["input.md"],"sourcesContent":["abcdefghijkl"],"names":[],"mappings":"AAAA,CAAC,CAAC,CAAO,CAAC,CAAC"}
{"source":"input.md","generatedLine":1,"generatedColumn":0,"originalLine":1,"originalColumn":0,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":1,"originalLine":1,"originalColumn":1,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":2,"originalLine":1,"originalColumn":2,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":3,"originalLine":1,"originalColumn":9,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":4,"originalLine":1,"originalColumn":10,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":5,"originalLine":1,"originalColumn":11,"name":null}
-----------------------
{"version":3,"file":"output.md","sources":["input.md"],"sourcesContent":["abcdefghijkl"],"names":[],"mappings":"AAAA,GAAG,GAAM,CAAC"}
{"source":"input.md","generatedLine":1,"generatedColumn":0,"originalLine":1,"originalColumn":0,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":3,"originalLine":1,"originalColumn":3,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":6,"originalLine":1,"originalColumn":9,"name":null}
{"source":"input.md","generatedLine":1,"generatedColumn":7,"originalLine":1,"originalColumn":10,"name":null}
-----------------------

[mourner]

Sorry for the late response — the fix looks great to me! Although I don't fully understand what's going on in the mappings code in general, so it would be great to get another pair of eyes on this — @guybedford maybe?

[obenjiro]

Any news on this one?