Allow GC heap growth

In current GC implementation, we allocate memory of the given heap size, and allocate (and initialize) metadata for the whole heap all at once at startup. This causes heap initialization extremely slow (causing 70% of the startup time - measured by @igariano01)

This should be fixed when we rewrite GC to allow heap growth so that we only need to mmap and initialize a small heap. The rewrite is on the schedule along with Issue #12 (closed).

operation	time (μs)
after rodal_init_deallocate	90.726
before mu_main	6.838
before gc_init	73.149
after gc_init	15,065.736
after init_runtime	35.896
after restore gc types	416.741
after build table	6,249.104
after loaded args	75.905
before swap_to_mu_stack	235.152
Total	22,249.247