Use the new `Pointer` and `Length` types in the Rust backend to emit
code that uses `*mut c_void` and `usize` instead of `i32` when working
with pointers and array lengths.

To represent `PointerOrI64`, use a `MaybeUninit<u64>`, since that type
can hold any `u64` and is documented to also preserve provenance.

This change happens to get the generated Rust code close to supporting
memory64, however it isn't complete; the abi code still emits hard-coded
`+ 4` offsets for loading the length of a pointer+length pair in memory.

And switch to `add` from `byte_add`.