TODO

v - implement beatiful registration for lifter functions
v - remove X-register search from dsl, and just add aliases for them in model
v - consider to remove explicit sign binary operations.
    as a positive effect - it eliminate syntax cases. as a negative it
    may introduce new bugs, because an expression (low word ra > zero)
    will not have a desired effect. so one will have to assign to a
    signed variable (low word ra) and then compare to zero.
x - implement one or few vector instructions to check that our model
    fits well to this purpose
 - add to model xer register
 - add to model floating point flags (or whatever there is)
v - consider update_link_register in branch insns - weird ?
v - add NOP insn just for full picture
v - add typecheck to tests
v - solve load/store error message - if we do need
v - check move instruction - that numbers, if we need to extend them.
    plus it would be useful to write a default clause for switch in
    move insns
v - add documentation
v - refactoring rtl, add [@@deriving ...]
v - take a look at lifters in part of signed/unsigned