* wasmtime-c-api: Only drop non-null `*mut wasm_ref_t`s * wasmtime-c-api: Handle null refs in `wasm_val_t` to `Val` conversion * wasmtime-c-api: Don't unwrap and rewrap `Option`s The `unwrap` can panic, and there isn't any point to this unwrap+rewrap. * wasmtime-c-api: Add conversions between `funcref` and `wasm_func_t` * wasmtime-c-api: More ownership documentation for `wasmtime.h`