Make size, decode and encode derives work with non-suffixed literals

[CaioSym]

Issues:

Resolves #114

Description of changes:

Implemented parsing for the repr attribute when deriving mls codec and used it to infer the type of the enum discriminants that don't have explicit suffixes.

Call-outs:

I'm very much a noob when it comes to rust macros. The code works but it genuinely feels like there should be an easier way to do this: Specifically, these two areas could use some insight for someone who knows what they are doing:

• Presumably there is a way to grab the repr values in FromDeriveInput directly but nothing I tried was working in a way that supported optionality and I did not want to force all declarations to define a repr.
• The code creating the actual suffixed literal is using some cheeky string interpolation because I could not for the love of me figure out a better way to simply add a suffix to the Literal being parsed.

Testing:

I've added a TestEnumWithoutSuffixedLiterals to the macro usage to ensure this works. The TestEnum case is also still working.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[tomleavy]

Thanks @CaioSym , just need to fix the clippy warnings

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.11765% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 89.34%. Comparing base (650ed8d) to head (ef607a6).

Files	Patch %	Lines
mls-rs-codec-derive/src/lib.rs	94.11%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #115   +/-   ##
=======================================
  Coverage   89.34%   89.34%           
=======================================
  Files         173      173           
  Lines       31333    31372   +39     
=======================================
+ Hits        27993    28029   +36     
- Misses       3340     3343    +3     

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