chore(c-api) Formalize API prefixes.

The new rule is the following:

* `wasm_` for the standard C API,
* `wasmer_` or `wasi_` for the Wasmer non-standard C API.

For all symbols inside the `unstable` module, the renaming `wasm_` to
`wasmer_` is done without deprecations. It was clear that those API
were unstable.

For all the other symbols, symbols have been renamed to `wasmer_` but
the old symbols have been kept with deprecation warnings.

Special note: The `wasm_named_extern_t` type (and associated
functions) was in `wasi` by mistake. Its place was in the `unstable`
module. This patch also fixes that.

The `wasm_declare_vec_*` macros have been updated to support a default
prefix, or a user-defined prefix. It's now possible to write
`wasm_declare_boxed_vec!(foo);` to get all the API prefixed by `wasm_`
(as previously), or `wasm_declare_boxed_vec!(foo, wasmer);` to prefix
with `wasmer_`.

A user not using symbols from the `unstable` module will continue to
get working code, modulo some deprecations, after this patch.

lib/c-api/src/wasm_c_api/unstable/mod.rs

@@ -1,3 +1,6 @@
 pub mod engine;
 pub mod module;
 pub mod target_lexicon;
+
+#[cfg(feature = "wasi")]
+pub mod wasi;