Creaation and destruction of MuVM, MuCtx and MuIRBuilder. Now the
`mvm->new_context`, `ctx->new_ir_builder`, `ctx->close_context` and
`b->abort` works. MuVM can also be created via a top-level "C"-style
exported function `mu_fastimpl_new()` so that in theory, it can be used
in C programs.