* Allow returning structs if copy
This commit does three things:
1. enables marshalling of structs as return args from interface funcs
but so far *only* for the case when the struct itself is copy
2. puts bits that use `std::convert::TryInto` in a local scope to avoid
multiple reimports
3. for added clarity, we now print for which `tref` type the marshalling
of results is unimplemented
The first case (1.) is required to make `fd_fdstat_get` WASI interface
func work which returns `Fdstat` struct (which is copy). The second
case (2.) caused me some grief somewhere along the lines when I was
playing with snapshot1. Putting the code that requires it inside a local
scope fixed all the issues
* Add proptests for returing struct if copyable
* Use write_ptr_to_guest to marshal value to guest
* Successfully return non-copy struct
c8ea27553d