HHBC AST type substitutions for friendly C interop
Summary:
We are working towards making the HHBC AST completely `repr(C)`. This diff makes some substitutions for builtin slices with substitutions supplied by the newly developed `ffi` crate (https://www.internalfb.com/diff/
D29466554 (https://github.com/facebook/hhvm/commit/
e843aa208e3fff8465ebe5980d5f59937cfbee70)).
## What's covered here
- New conversion functions for `Slice` and `Str`
- Expressed in terms of `Slice`, `Str` and `repr(C)`:
- `ParamId<>`
- `hhbc_by_ref_id::class::Type<>` and friends,
- `ByRefs<'arena>`
Reviewed By: shiqicao
Differential Revision:
D29486892
fbshipit-source-id:
2217ccfe1bb4e627a20bdab5839ea5f40af8a5ad