cbindgen warnings

I found this interesting

```
WARN: Skip wasmvm::CONFIG_ARG - (Unsupported literal expression. Lit::Str { token: "config" })
WARN: Skip wasmvm::CACHE_ARG - (Unsupported literal expression. Lit::Str { token: "cache" })
WARN: Skip wasmvm::WASM_ARG - (Unsupported literal expression. Lit::Str { token: "wasm" })
WARN: Skip wasmvm::CHECKSUM_ARG - (Unsupported literal expression. Lit::Str { token: "checksum" })
WARN: Skip wasmvm::GAS_REPORT_ARG - (Unsupported literal expression. Lit::Str { token: "gas_report" })
WARN: Skip wasmvm::ARG1 - (Unsupported literal expression. Lit::Str { token: "arg1" })
WARN: Skip wasmvm::ARG2 - (Unsupported literal expression. Lit::Str { token: "arg2" })
WARN: Skip wasmvm::ARG3 - (Unsupported literal expression. Lit::Str { token: "arg3" })
WARN: Passing zero-sized struct cache_t across the FFI boundary is undefined behavior
WARN: Skip wasmvm::CYBERPUNK - (not `no_mangle`).
WARN: Skip wasmvm::MEMORY_CACHE_SIZE - (not `pub`).
WARN: Skip wasmvm::MEMORY_LIMIT - (not `pub`).
WARN: Skip wasmvm::GAS_LIMIT - (not `pub`).
WARN: Skip wasmvm::VERSION - (not `no_mangle`).
faddat@Jacobs-MacBook-Pro ~/w/libwasmvm (faddat/grpc)>
```