This might be a big diff, but I _hope_ that it does not change much
functionally (hopefully it changes nothing generation specific).
What has changed?
-----------------
- I had to merge the three crates into one, to allow `macros` to impl
functions on Types defined in `parser`.
- As mentioned in the point above, the conversion function are now
inherent to the type they convert (i. e. `r#type.to_rust()` instead of
`type_to_rust(r#type)`).
- The conversion function have been sorted, by what they convert to:
- `to_rust()` converts a type to be used in rust *host* code.
- `to_c()` converts a type to be used in c *host* code.
- `to_auxiliary_c()` converts a type to be used in c *auxiliary*
code.
- The top-most `generate` method of `TrixyConfig` now returns a
`FileTree` instead of writing the files directly. The `FileTree` can
still be materialize with the `materialize` method. But this change
facilitates moving non-generation focused code out of the `generate`
method.
What is the difference between _host_ and _auxiliary_ code?
-----------------------------------------------------------
Auxiliary code is always written in the language it is generated for. So
auxiliary code for c would be written in c (or at least in a subset of c),
as it represents c header files.
Host code is always written in rust. This is the code that is
responsible for implementing the actual ffi stuff. In our case these are
the `extern "C"` functions and the types, defined in trixy.
The rust host code is generating the native rust representations of
these types.
