FFI types (after conversion from Rust types) do not match the WASM types in spec

[bhargavbh]

this is somewhat linked to #522
It would be nice to clear up how the types in the WASM function signatures of the spec should be interpreted. Right now, we sometimes use u32/u64 in the function signature which are not really primitive types in WASM. In most cases, the function signature has i32/i64, but the FFI types (after conversion from Rust) are unsigned u32/u64 and have to be interpreted unsigned in WASM by relevant operators. Definitions 193, 194 might be the appropriate place to add this clarification.