This commit restructures some of the internals of `wasm-gc` now that
I've actually got a better grasp on the wasm format and what all the
ownership edges look like. This shouldn't actually result in any
user-facing changes, but should make us be a bit more compatible with
operations in the future.
Memories/tables/elements/segments are no longer considered automatic
roots but rather need to be rooted by something else to prevent a gc.
For example an element section is gc'd along with a table if the table
is never referenced, along with data segments as well if the memory
isn't referenced.
Additionally all index sets now don't contained offseted indices, but
rather everything is always stored relative to the "index space" to
ensure consistency.
This should make it a bit easier to add future items to gc!
The `wasm-bindgen` crate is effectively the only user of this crate now
that the `wasm-gc` tool has been deprecated. It's also much easier to
keep it in this repository as it's easier to sync changes to
`parity-wasm`. I'd also like to start refactoring out utilities for
managing a `parity_wasm::Module` to share between this crate and the
other CLI support code.
