Support allocation into a non-moving space

[udesou]

Some objects such as types, and type names, which are used as metadata when scanning objects, should be allocated into a non-moving space. While MMTk already provides an API for doing so, and there's already a PR to add this support to Julia, MMTk currently allocates these objects in the immortal space; For this reason, they never get collected and increase memory consumption leading to OOM errors in the CI (#101). This issue is to keep track of this problem, which should be solved after we are able to allocate such objects in a proper non-moving space, such as marksweep (which has a few issues in mmtk-core at the moment: mmtk/mmtk-core#688).

NB: In the moving immix version (#93) the non-moving characteristic has been achieved by pinning these objects indefinitely.