Make bincode_derive 0 dependencies

[VictorKoenders]

This PR removes syn, proc_macro2 and quote from bincode_derive.

We still have to keep proc_macro2 as a dev dependency as it is otherwise impossible to test the parser. This is automatically used instead of proc_macro when running tests.

The change is roughly split into 2 parts:

Parsing

Most of parsing is done in the derive/src/parse/ folder. This will generate the following types:

• Visibility, not being used currently
• DataType either Struct or Enum, with the name of the data type being parsed
• Generics the generics part of the type, e.g. struct Foo<'a>
• GenericConstraints the "where" part of the type

Generate

Generating the code implementation is done in either derive/src/derive_enum.rs and derive/src/derive_struct.rs.

This is supported by the structs in derive/src/generate. The most notable points of this module are:

• StreamBuilder is a thin but friendly wrapper around TokenStream
• Generator is the base type of the code generator. This has helper methods to generate implementations:
  • ImplFor is a helper struct for a single impl A for B construction. In this functions can be defined:
    • GenerateFnBody is a helper struct for a single function in the above impl. This is created with a callback to FnBuilder which helps set some properties. GenerateFnBody has a stream() function which returns StreamBuilder for the function.

The code is tested against the current cases in tests/derive.rs, however we'll need a lot more test cases before this PR can be merged.

For testing purposes, all generated code is outputted to the current target folder, under file name <struct/enum name>_Encodeable.rs and <struct/enum name>_Decodeable.rs. This can help with debugging.

[codecov-commenter]

Codecov Report

Merging #409 (6662918) into feature/deserde (cb23078) will decrease coverage by 21.19%.
The diff coverage is 44.87%.

@@                 Coverage Diff                  @@
##           feature/deserde     #409       +/-   ##
====================================================
- Coverage            87.68%   66.48%   -21.20%     
====================================================
  Files                   16       29       +13     
  Lines                  958     1886      +928     
====================================================
+ Hits                   840     1254      +414     
- Misses                 118      632      +514     

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/generate_fn.rs	0.00% <0.00%> (ø)
derive/src/generate/generator.rs	0.00% <0.00%> (ø)
derive/src/generate/impl_for.rs	0.00% <0.00%> (ø)
derive/src/generate/stream_builder.rs	0.00% <0.00%> (ø)
tests/derive.rs	100.00% <ø> (ø)
derive/src/lib.rs	7.69% <7.69%> (ø)
derive/src/error.rs	12.50% <13.79%> (ø)
derive/src/parse/generics.rs	66.94% <66.94%> (ø)
... and 6 more

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[ZoeyR]

Looks like this parser can't handle field attributes (i.e. #[serde(skip)]). It shouldn't be too bad to add them because as far as I can tell they can all be ignored.