Import return buffers are never freed

**Issue details assume #319 is merged.**

Import return buffers allocated via `cabi_realloc` in the splicer are never freed. The old `free_list` bulk free in `post_call` was the only mechanism reclaiming them, but it also caused use-after-free (#224, fixed by #319).

The retptr is returned to JS as a pointer, and JS reads from it after the WASM glue returns, so there's no obvious place to insert a free in the current architecture. This is an unbounded leak for components that make repeated import calls returning strings or lists.

Separately, I noticed the bindgen sets `post_return: None` so ComponentizeJS never generates post-return functions for exports. The canonical ABI expects the host to call post-return so the guest can free export return buffers. I haven't investigated whether `post_call` covers this already.