Bytes const size borrowed arrays

[matix2267]

Currently Bytes (added in #277) supports some const size array types (e.g. [u8; N] and Box<[u8; N]>)
but not others (e.g. &[u8; N] and Cow<'_, [u8; N]>).

This pull request implements Bytes support for &[u8; N] and Cow<'_, [u8; N]>
This bring parity between dynamic and const sized types.

Before:

Sizing	Owned	Borrowed	Cow	Box
Const	✅ [u8; N]	❌ &[u8; N]	❌ Cow<'_, [u8; N]>	✅ Box<[u8; N]>
Dynamic	✅ Vec<u8>*	✅ &[u8]	✅ Cow<'_, [u8]>	✅ Box<[u8]>

After:

Sizing	Owned	Borrowed	Cow	Box
Const	✅ [u8; N]	✅ &[u8; N]	✅ Cow<'_, [u8; N]>	✅ Box<[u8; N]>
Dynamic	✅ Vec<u8>*	✅ &[u8]	✅ Cow<'_, [u8]>	✅ Box<[u8]>

I've also added a separate commit with an example of using fully borrowed types, but that example only does serialization
because RON doesn't support 0-copy deserialization for byte arrays (as it needs to decode them from base64).
Because of that I'm not sure if this example is useful so feel free to merge only the main commit.

_{* Technically [u8] corresponds to [u8; N] but [u8] is an unSized type and cannot be
directly used in structs.}

[codecov]

Codecov Report

Merging #325 (3130c38) into master (bb1c80a) will decrease coverage by 1.54%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
- Coverage   79.39%   77.85%   -1.55%     
==========================================
  Files          45       45              
  Lines        2577     2628      +51     
==========================================
  Hits         2046     2046              
- Misses        531      582      +51     

Impacted Files	Coverage Δ
src/de/const_arrays.rs	36.73% <0.00%> (-36.74%)	⬇️
src/lib.rs	75.00% <ø> (ø)
src/ser/const_arrays.rs	86.66% <0.00%> (-13.34%)	⬇️

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[jonasbb]

Thank you. This definitely is an oversight of mine. For serialization &Bytes should work. I'm unsure if that would support 0-copy deserialization, though. The missing implementation on Cow<'_, [u8; N]> is a problem :)
It is great that you added the extra documentation.
I can create a new bugfix release later with those changes.

[jonasbb]

bors r+

[bors]

Build succeeded:

• Rustsec Audit
• Tests result