wasmtime-c-api: Add support for externref values

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

crates/c-api/src/global.rs

@@ -1,5 +1,6 @@
 use crate::{handle_result, wasmtime_error_t};
 use crate::{wasm_extern_t, wasm_globaltype_t, wasm_store_t, wasm_val_t};
+use std::mem::MaybeUninit;
 use std::ptr;
 use wasmtime::{Extern, Global};
 
@@ -72,8 +73,10 @@ pub extern "C" fn wasm_global_type(g: &wasm_global_t) -> Box<wasm_globaltype_t>
 }
 
 #[no_mangle]
-pub extern "C" fn wasm_global_get(g: &wasm_global_t, out: &mut wasm_val_t) {
-    out.set(g.global().get());
+pub extern "C" fn wasm_global_get(g: &wasm_global_t, out: &mut MaybeUninit<wasm_val_t>) {
+    unsafe {
+        ptr::write(out.as_mut_ptr(), wasm_val_t::from_val(g.global().get()));
+    }
 }
 
 #[no_mangle]