feat(serde_with): add with_suffix!

[fgardt]

This should properly close #381.

Apart from the used example it is pretty much identical to the existing with_prefix! code, except for the few places that actually do the suffixing / trimming.

[codecov]

Codecov Report

Attention: Patch coverage is 56.33803% with 62 lines in your changes missing coverage. Please review.

Project coverage is 66.91%. Comparing base (2d70b70) to head (2b4658c).
Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
serde_with/src/with_suffix.rs	56.33%	62 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
- Coverage   67.34%   66.91%   -0.43%     
==========================================
  Files          40       41       +1     
  Lines        2468     2639     +171     
==========================================
+ Hits         1662     1766     +104     
- Misses        806      873      +67     

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[jonasbb]

Thank you for doing the work of adding with_suffix!. Since it is mostly a copy, I don't see a problem here. I like that you added a proper new example to the documentation :)

[fgardt]

any ETA for a new release that would contain this? @jonasbb