feat(c-api) Allow extern types to own data.

We have known memory leaks with extern types. The idea is to change
the code so that extern types can hold/own data. This patch does that.

A `wasm_externtype_t` holds a `WasmExternType` enum. This enum owns
sibling types such as `WasmFunctionType`, `WasmGlobalType`,
`WasmTableType` and `WasmMemoryType`. It is those structures that ows
the extern types data, like `params` and `results` as
`wasm_valtype_vec_t` for `WasmFunctionType`. That way, for example,
`wasm_functype_t` can return a pointer to these vec which it owns.

A `wasm_externtype_t` continues to be transmuted to `wasm_functype_t`
etc. Nothing changes on that side.

lib/c-api/src/wasm_c_api/macros.rs

@@ -17,6 +17,7 @@ macro_rules! wasm_declare_vec_inner {
 macro_rules! wasm_declare_vec {
     ($name:ident) => {
         paste::item! {
+            #[derive(Debug)]
             #[repr(C)]
             pub struct [<wasm_ $name _vec_t>] {
                 pub size: usize,
@@ -108,6 +109,7 @@ macro_rules! wasm_declare_vec {
 macro_rules! wasm_declare_boxed_vec {
     ($name:ident) => {
         paste::item! {
+            #[derive(Debug)]
             #[repr(C)]
             pub struct [<wasm_ $name _vec_t>] {
                 pub size: usize,