Remove `LLVMRustCoverageHashCString`
Coverage has two FFI functions for computing the hash of a byte string. One takes a ptr/len pair (`LLVMRustCoverageHashByteArray`), and the other takes a NUL-terminated C string (`LLVMRustCoverageHashCString`).
But on closer inspection, the C string version is unnecessary. The calling-side code converts a Rust `&str` into a `CString`, and the C++ code then immediately turns it back into a ptr/len string before actually hashing it. So we can just call the ptr/len version directly instead.
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This PR also fixes a bug in the C++ declaration of `LLVMRustCoverageHashByteArray`. It should be `size_t`, since that's what is declared and passed on the Rust side, and it's what `StrRef`'s constructor expects to receive on the callee side.
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