Allow returning structs if copy (#19)

* 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

tests/structs.witx

@@ -29,4 +29,12 @@
     (param $an_pair $pair_int_and_ptr)
     (result $error $errno)
     (result $double s64))
+  (@interface func (export "return_pair_ints")
+    (result $error $errno)
+    (result $an_pair $pair_ints))
+  (@interface func (export "return_pair_of_ptrs")
+    (param $first (@witx const_pointer s32))
+    (param $second (@witx const_pointer s32))
+    (result $error $errno)
+    (result $an_pair $pair_int_ptrs))
 )