[Epic] Pure C API

[amomchilov]

Shopify is actively working on decoupling the RBS parser from its Ruby bindings, in order to expose a new pure C API.

This API will be more compatible (e.g. it can integrate into JRuby, Truffle, Sorbet, etc.), and will allow for multi-threaded RBS parsing (since it won't be blocked by Ruby's GVL).

Ready to review PRs

This is still a work-in-progress, but this list will be updated to list all PRs that are ready to review, in the order that they should be merged.

1. Add templating mechanism to automatically generate .c/.h files #2098
2. Clean up temporary workarounds from templated code #2102
3. Un-hardcode repo name in test suite #2128
4. Designated initializers #2131
5. Remove forward declarations of local variables #2132
6. Extract out usages of Ruby IDs #2175

CC @Morriar, @soutaro