Constant folding and propagation in word_simp

[myreen]

This is @AndreasLoow's optimisation for wordLang. This optimisation pushes constants forward and performs simple constant folding. The optimisation is able to push constants past Call, if the call does not have an exception handler.

[AndreasLoow]

We can probably close #129 when this is merged.

[xrchz]

Trying to build this failed in the bootstrap translation:

 Translating word_simp_apply_if_opt
 Saved theorem _____ "word_simp_apply_if_opt_v_thm"
 Translating word_simp_simp_if
 Translating word_simp_const_fp_move_cs
 error in quse /home/ramana/cakeml5/compiler/bootstrap/translation/to_word64ProgScript.sml : UnsupportedType ?
 error in load to_word64ProgScript : UnsupportedType ?
 Failed translation: const_fp_inst_cs
 Failed translation: const_fp_loop
 Failed translation: const_fp
 Failed translation: compile_exp

If I find time I'll look into it, but anyone else can feel free to fix too.

[xrchz]

@tanyongkiam I think this might require some of your expertise

[tanyongkiam]

It looks like an uninstantiated (word) type variable. I've left it building in the background.