changed a condition on coealescing. Fixed a bug in spilling rewrite.
Currently test_extern_Func is broken

was using nalgebra::DMatrix to implement a graph

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

refactoring; use SPAWN_PROC environment variable to determine whether to spawn child process or not. Not spawning child process may be able to provide clearer assertion fails

introduced Move instruction at IR level.
Intermediate blocks will be inserted for multi-target branch
instructions

internally int1 is int8