refactor mallocarray_t to reflect it should carry jl_genericmemory_t

[d-netto]

Seems like this mallocarray_t container is only carrying objects of type jl_genericmemory_t after the memory work.

Let's:

• Rename the container type to mallocmemory_t.
• Make the payload type a bit more strict to ensure a bit more type safety (i.e. change the a field from jl_value_t * to jl_genericmemory_t *).
• Add a comment to indicate the pointer tagging in the lowest bit of the payload.

[d-netto]

Will merge since the only failure in x86_64-linux-gnuassertrr-net seems unrelated (and this test is currently flaky).