functions to enable encoding/decoding serde types

[VictorKoenders]

This is an initial attempt at supporting serde in bincode 2.

I did have to make separate functions for all serde-related functionality. This is possibly preventable.

Implementing Serialize was relatively easy.

For Deserialize and DeserializeOwned I ended up making 2 functions, which support BorrowDecoder and Decoder respectfully. These also have 2 separate functions right now, we might be able to merge this into one function.

[codecov-commenter]

Codecov Report

Merging #422 (4f1d5de) into trunk (5cbd7fd) will decrease coverage by 6.41%.
The diff coverage is 20.67%.

@@            Coverage Diff             @@
##            trunk     #422      +/-   ##
==========================================
- Coverage   71.04%   64.62%   -6.42%     
==========================================
  Files          41       47       +6     
  Lines        2811     3237     +426     
==========================================
+ Hits         1997     2092      +95     
- Misses        814     1145     +331     

Impacted Files	Coverage Δ
serde.rs	0.00% <0.00%> (ø)
src/de/decoder.rs	42.85% <ø> (-28.58%)	⬇️
src/de/impls.rs	75.00% <0.00%> (-0.59%)	⬇️
src/enc/encoder.rs	57.14% <ø> (-14.29%)	⬇️
src/enc/impls.rs	89.06% <0.00%> (-1.90%)	⬇️
src/error.rs	0.00% <0.00%> (ø)
src/features/impl_std.rs	71.75% <ø> (ø)
src/features/serde/mod.rs	0.00% <0.00%> (ø)
src/features/serde/ser.rs	20.20% <20.20%> (ø)
src/features/serde/de_borrowed.rs	21.50% <21.50%> (ø)
... and 8 more

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[ZoeyR]

I feel like this should have an impl Encode for T where T: Serialize and same for Decode. With the current implementation you can't bring a struct that only implements Serialize into a struct you want to implement Encode on

[VictorKoenders]

I've opened a separate issue for having bincode Decode/Encode work together with serde's traits here: #433