This required that `wasm_val_t` have a `Drop` implementation, an explicit
`Clone` implementation, and no longer be `Copy`, which rippled out through the
crate a bit.
Additionally, `wasm_func_call` and friends were creating references to
uninitialized data for its out pointers and assigning to them. As soon as
`wasm_val_t` gained a `Drop` impl and tried to drop the old value of the
assignment (which is uninitialized data), then things blew up. The fix is to
properly represent the out pointers with `MaybeUninit`, and use `ptr::write` to
initialize them without dropping the old data.
Part of #929
4cdf8b7cfd