Remove HostRef<T> from the C API (#1926)

This commit removes `HostRef<T>` from the C API which only served the
purpose now of converting each type to a `wasm_ref_t*`. Our
implementation, however, does not guarantee that you'll get the same
`wasm_ref_t*` for each actual underlying item (e.g. if you put a func in
a table and then get the func as an export and from the table then
`same` will report `false`). Additionally the fate of `wasm_ref_t*`
seems somewhat unclear at this point.

The change here is to make the `same` and cast functions all abort
saying they're unimplemented. (similar to the host info functions). If
and when we get around to reimplementing these functions we can ensure
they're implemented uniformly and work well for all intended use cases.

examples/memory.c

@@ -185,7 +185,6 @@ int main(int argc, const char* argv[]) {
 
   // Try cloning.
   wasm_memory_t* copy = wasm_memory_copy(memory);
-  assert(wasm_memory_same(memory, copy));
   wasm_memory_delete(copy);
 
   // Check initial memory.