Parse -0 as float -0.0 instead of integer 0

[dtolnay]

Fixes #799.

[silverjam]

This seems to break a "round trip" of a 0 value (since it seams 0, not just -0 will get parsed as -0.0). We have a test that does a conversion from JSON -> typed structure then back to JSON, when we compare the reference JSON to the generated JSON we get a breakage because 0 is not equal to -0.

---- test_json2json stdout ----
input: "/home/ubuntu/dev/libsbp/test_data/roundtrip.json2json.output", output: "/home/ubuntu/dev/libsbp/test_data/test_json2json.output.third_transform"
value mismatch at line 0
json atoms at path ".acc[1]" are not equal:
    lhs:
        0

    rhs:
        -0.0