Lossless Value::Number and allow +unsigned

[juntyr]

This PR

• cleans up the value module a bit
• expands the value::Number enum to cover all possible number types losslessly
• merges the private parser::AnyNum type with value::Number
• fixes plus sign on unsigned integers #445 by allowing +unsigned to parse as an unsigned integer

I see this as a small improvement towards a better Value. While it does touch a lot of tests, since Number is more explicit now and always chooses the smallest variant that fits (and serde's Deserialize impls do the rest by upcasting again if necessary), more control could be given to the user with #241, if someone wants to implement that.

• I've included my change in CHANGELOG.md

[codecov-commenter]

Codecov Report

Patch coverage: 75.78% and project coverage change: -2.36% ⚠️

Comparison is base (dea68fe) 85.28% compared to head (c3af695) 82.93%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   85.28%   82.93%   -2.36%     
==========================================
  Files          72       74       +2     
  Lines        8850     9257     +407     
==========================================
+ Hits         7548     7677     +129     
- Misses       1302     1580     +278     

Files Changed	Coverage Δ
src/value/map.rs	46.26% <46.26%> (ø)
src/de/value.rs	74.06% <63.07%> (-14.46%)	⬇️
src/value/mod.rs	32.36% <66.66%> (-14.90%)	⬇️
src/value/number.rs	71.27% <71.27%> (ø)
src/parse.rs	75.31% <72.88%> (-3.87%)	⬇️
src/de/mod.rs	74.90% <100.00%> (-1.14%)	⬇️
src/de/tests.rs	88.35% <100.00%> (-11.65%)	⬇️
src/ser/tests.rs	86.30% <100.00%> (-13.70%)	⬇️
src/ser/value.rs	30.76% <100.00%> (-19.24%)	⬇️
tests/115_minimal_flattening.rs	100.00% <100.00%> (ø)
... and 6 more

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