* core: add temporary, ruffle-internal copy of `gc-arena` crate
This will allow bumping the upstream `gc-arena` version while
reexporting our own version of the old `GcCell` API, so that
Ruffle's code can be gradually migrated.
Once the migration is done, this crate should be removed.
* core: bump `gc-arena` to kyren/gc-arena#56
Add back the removed `GcCell` to our internal facade crate
* core: bump `gc-arena` to current master
This bump renames `Gc::allocate` to `Gc::new`
* core: rename `GcCell::allocate` to `GcCell::new`, to match `Gc`
* core: bump gc-arena to (slighly after) v0.3.1
Add typedefs for old `*Context` names in the gc-arena facade crate
* core: replace uses of `CollectionContext<'_>` by `&Collection`
* core: Add `gc()` convenience method for `*Context` and `Activation` types
This allows shortening most instances of `[activation.]context.gc_context`
to `activation.gc()` or `context.gc()` (but not all instances, because of
borrowck) Note that this doesn't actually do these shortenings to avoid
major code churn.
TÖRÖK Attila