Clean up the ML code for: type_compare mutability_compare prim_compare

[barockobamo]

From #26 :

clean up the ML code for: type_compare mutability_compare prim_compare;
for prim_compare, we should be able to match on pairs of arguments, with a catch-all

[IgnoredAmbience]

Matching on pairs of arguments is currently outside of our ml subset. Make direct comparison of these compare functions with ecma262 text to see how we should implement to be syntactically close.