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now we have 4 passes: fast_inst_sel, fast_reg_alloc, followed
by slow_inst_sel and slow_reg_alloc. This might make the
compilation pipeline more linear

partially serialize compiled function into bootimage

cannot get it work by loading it dynamically. Rust mangles mu runtime
functions in test executable (even if I stated #[no_mangle]), so the
dynamic library cannot call mu runtime functions (symbols not found)

(for blocks created during instruction selection)

gonna implement THREADEXIT

The register allocation is not optimal (liveness analysis may be
inaccurate).