Implemented ron::value::RawValue

[juntyr]

Adds the ron::value::RawValue type, inspired by serde-rs, which holds a valid RON fragment string and can be (de)serialised inside any container.

• I've included my change in CHANGELOG.md

[codecov-commenter]

Codecov Report

Merging #407 (c4d05b3) into master (a63b3c8) will increase coverage by 1.02%.
The diff coverage is 74.01%.

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
+ Coverage   78.74%   79.76%   +1.02%     
==========================================
  Files          51       54       +3     
  Lines        6737     7068     +331     
==========================================
+ Hits         5305     5638     +333     
+ Misses       1432     1430       -2     

Impacted Files	Coverage Δ
src/error.rs	36.36% <0.00%> (+0.06%)	⬆️
src/value/mod.rs	67.47% <ø> (ø)
src/ser/raw.rs	5.21% <5.21%> (ø)
src/value/raw.rs	99.20% <99.20%> (ø)
src/de/mod.rs	76.02% <100.00%> (+2.10%)	⬆️
src/parse.rs	79.53% <100.00%> (ø)
src/ser/mod.rs	74.63% <100.00%> (-3.64%)	⬇️
tests/407_raw_value.rs	100.00% <100.00%> (ø)
... and 3 more

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[juntyr]

?r @torkleyy I stumbled across this API some time ago and thought it would be quite useful to also have it inside ron. While I added some convenience functions to RawValue for the common cases, I decided against implementing Serializer and Deserializer directly as those would only duplicate functionality which is already available by just using the existing Deserializer with the &str one can get from RawValue.

[torkleyy]

Makes sense & looks good to me :)