Add support for XML Schema xs:list type

[Mingun]

This PR allows deserializing Vec and other sequences from space-separated lists, as defined by XML Schema specification. So now this is possible:
Deserialize

struct AnyName {
  // Will contain vec!["first", "second", "third"]
  attribute: Vec<String>,
}

from

<any-tag attribute="first second third">

[codecov-commenter]

Codecov Report

Merging #395 (328b4f6) into master (0a42987) will increase coverage by 1.71%.
The diff coverage is 86.05%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
+ Coverage   60.79%   62.50%   +1.71%     
==========================================
  Files          19       20       +1     
  Lines        9843    10664     +821     
==========================================
+ Hits         5984     6666     +682     
- Misses       3859     3998     +139     

Flag	Coverage Δ
unittests	62.50% <86.05%> (+1.71%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/errors.rs	12.50% <0.00%> (-0.41%)	⬇️
src/events/mod.rs	73.87% <ø> (ø)
src/de/map.rs	72.85% <51.16%> (-15.90%)	⬇️
src/de/simple_type.rs	90.13% <90.13%> (ø)
src/de/mod.rs	78.07% <100.00%> (+1.99%)	⬆️
src/reader.rs	88.10% <100.00%> (-0.12%)	⬇️
src/de/escape.rs	23.35% <0.00%> (-43.07%)	⬇️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0a42987...328b4f6. Read the comment docs.

[dralley]

I tried to do a code review on this PR but since I don't use quick-xml via serde, am not familiar with Serde's parsing, etc. I realized I don't have time to do all of that research, plus dig through the XML spec, plus review the PR, and keep everything in my head long enough to get it done.

So I have just given this a brief look through. You've done a great job with the testing

[dralley]

Feel free to merge this without a full approval