add missing Bofbool case in Const_eval

[redianthus]

Hi @mlemerre !

We used this operation in Owi with @S41d and noticed it was missing when we started getting runtime assertion failures.

The only difference at runtime is:

| TC.Bofbool _size -> if boolean a then Z.one else Z.zero

I also had to add type annotations because of the polymorphic recursion, otherwise it would complain with:

$ dune build @install -p codex
File "terms/const_eval.ml", line 37, characters 19-20:
37 | if boolean a then Z.one else Z.zero
                ^
Error: The value a has type Operator.Function_symbol.boolean Terms.t
but an expression was expected of type 'a
This instance of Operator.Function_symbol.boolean is ambiguous:
it would escape the scope of its equation

While adding the type annotations, I realized the big mutual-recursion group was not needed and most functions could be moved out of it. Only the boolean and binary cases are mutually recursive. So I went ahead and decided to split the group in small functions, I think this is easier to read like this.

Best,