Implement lowering and intrinsics of `Int64,Float{32,64}` in the Wasm backend

Currently, the typed IR has those primitive types in the erased type hierarchy, but lowering of these types to their Wasm counterparts (`i64`, `f{32,64}`) are not implemented. Moreover, some intrinsics will also need to be implemented to exercise these types in the difftests.

See https://github.com/hkust-taco/mlscript/pull/504#issuecomment-5365514010.