This further helps with the potential denial of service problem in
issue #2782 / oss-fuzz#49462 / oss-fuzz#20177.

Instead of allocating a new `GVariant` for each nesting level of
maybe-types, allocate a single `GVariant` and give it the fully-nested
maybe type as its type. This has to be done in serialised form.

This prevents attackers from triggering O(size of container × typedecl
depth) allocations.

This is a follow up to commit 3e313438

,
and includes a test.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Fixes: #2782
oss-fuzz#20177
oss-fuzz#49462