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.
