split off BorrowDecode from Decode in bincode_derive

[VictorKoenders]

We've discussed splitting off Decode from BorrowDecode derive macro before. This PR actualizes that.

This should offer people a solution to issues where the type implements BorrowDecode while people are explicitly requesting Decode. It does not fix the issue around Cow<T> always needing Decode, we can't fix that until specialization implementation.

[codecov-commenter]

Codecov Report

Merging #432 (dfdc924) into trunk (4120069) will decrease coverage by 0.48%.
The diff coverage is 8.80%.

@@            Coverage Diff             @@
##            trunk     #432      +/-   ##
==========================================
- Coverage   71.04%   70.55%   -0.49%     
==========================================
  Files          41       42       +1     
  Lines        2811     2839      +28     
==========================================
+ Hits         1997     2003       +6     
- Misses        814      836      +22     

Impacted Files	Coverage Δ
derive/src/derive_enum.rs	0.00% <0.00%> (ø)
derive/src/derive_struct.rs	0.00% <0.00%> (ø)
derive/src/generate/generator.rs	0.00% <ø> (ø)
derive/src/lib.rs	5.33% <0.00%> (-2.08%)	⬇️
src/lib.rs	100.00% <ø> (ø)
tests/derive.rs	100.00% <ø> (ø)
tests/utils.rs	95.23% <ø> (ø)
src/features/impl_alloc.rs	100.00% <100.00%> (ø)
tests/alloc.rs	100.00% <100.00%> (ø)
tests/issues/issue_431.rs	100.00% <100.00%> (ø)
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4120069...dfdc924. Read the comment docs.

[VictorKoenders]

I've taken the liberty to also re-export the Decode and Encode trait in the root of the crate, as these are used often