Feature/improve serde

[VictorKoenders]

To be merged after #475

This PR improves serde support for alloc and no_std targets. Until #472 is stabilized, we'll have to make do with this.

I also took the liberty to move all of serde's errors to EncodeError::Serde and DecodeError::Serde, so they don't pollute the main error too much.

[codecov-commenter]

Codecov Report

Merging #477 (81d5d1c) into trunk (d8ba21a) will decrease coverage by 0.27%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##            trunk     #477      +/-   ##
==========================================
- Coverage   66.44%   66.17%   -0.28%     
==========================================
  Files          38       38              
  Lines        2691     2702      +11     
==========================================
  Hits         1788     1788              
- Misses        903      914      +11     

Impacted Files	Coverage Δ
src/error.rs	0.00% <ø> (ø)
src/features/serde/de_borrowed.rs	0.00% <0.00%> (ø)
src/features/serde/de_owned.rs	0.00% <0.00%> (ø)
src/features/serde/mod.rs	0.00% <0.00%> (ø)
src/features/serde/ser.rs	0.00% <0.00%> (ø)

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