Add wasmtime-specific C APIs for tables (#1654)

This commit adds a suite of `wasmtime_funcref_table_*` APIs which mirror
the standard APIs but have a few differences:

* More errors are returned. For example error messages are communicated
  through `wasmtime_error_t` and out-of-bounds vs load of null can be
  differentiated in the `get` API.

* APIs take `wasm_func_t` instead of `wasm_ref_t`. Given the recent
  decision to remove subtyping from the anyref proposal it's not clear
  how the C API for tables will be affected, so for now these APIs are
  all specialized to only funcref tables.

* Growth now allows access to the previous size of the table, if
  desired, which mirrors the `table.grow` instruction.

This was originally motivated by bytecodealliance/wasmtime-go#5 where
the current APIs we have for working with tables don't quite work. We
don't have a great way to take an anyref constructed from a `Func` and
get the `Func` back out, so for now this sidesteps those concerns while
we sort out the anyref story.

It's intended that once the anyref story has settled and the official C
API has updated we'll likely delete these wasmtime-specific APIs or
implement them as trivial wrappers around the official ones.

crates/c-api/include/wasmtime.h

@@ -254,6 +254,32 @@ WASM_API_EXTERN own wasmtime_error_t *wasmtime_module_validate(
     const wasm_byte_vec_t *binary
 );
 
+
+// Similar to `wasm_table_*`, except these explicitly operate on funcref tables
+// and work with `wasm_func_t` values instead of `wasm_ref_t`.
+WASM_API_EXTERN own wasmtime_error_t *wasmtime_funcref_table_new(
+    wasm_store_t *store,
+    const wasm_tabletype_t *element_ty,
+    wasm_func_t *init,
+    own wasm_table_t **table
+);
+WASM_API_EXTERN bool wasmtime_funcref_table_get(
+    const wasm_table_t *table,
+    wasm_table_size_t index,
+    own wasm_func_t **func
+);
+WASM_API_EXTERN own wasmtime_error_t *wasmtime_funcref_table_set(
+    wasm_table_t *table,
+    wasm_table_size_t index,
+    const wasm_func_t *value
+);
+WASM_API_EXTERN wasmtime_error_t *wasmtime_funcref_table_grow(
+    wasm_table_t *table,
+    wasm_table_size_t delta,
+    const wasm_func_t *init,
+    wasm_table_size_t *prev_size
+);
+
 #undef own
 
 #ifdef __cplusplus