some pointers to keep those Arc struct alive, so Drop doesnt get called

impl Drop for VM

Instead of using a cyclic doubly linked list, I am going to use two
doubly linked list as fromspace and tospace. This will greatly simplify
implementation.

make tracing able to trace freelist space (may have degraded
performance, gonna need to tune gc performance later)

1. see vm/vm_options.rs for usage and default values
2. added mu_fastimpl_new_with_opts under vm/api/api_impl/muvm.rs

cargo test needs ALWAYS run with 1 test thread (RUST_TEST_THREADS=1)

FIXME: GC settings such as heap size, n_gcthreads, should be properly serialized

gonna port tests to this repo