parsing: use the `CStr` type from `core`

[rmsyn]

Instead of wrapping a byte slice in the local CStr, wraps the core::ffi::CStr type.

Avoids duplication of effort on the core library, and should help future-proof for additions like the c"" feature for creating C-string literals.

Only includes the minimal changes, the local CStr wrapper can likely be removed with more changes.