compiler: replace disjunctions

Had to move the logic into a separate function to avoid stack overflows,
because we are recursing very large/complex functions. Other than that I
tried to preserve the existing logic; I will double-back and refactor in
a different commit.

I also removed all the unused code from AstElemExt::type_check, since it
would no longer compile -- I changed the disjunction code to take the
left/right weights as parameters rather than storing them in the
AstElemExt object.

The goal here is to remove the `branch_prob` field entirely, which will
then let us make AstElemExt immutable, which will unlock some further
refactorings. But until I'm done this, I have to just shuffle around
the existing logic and the result is pretty big/ugly.

Author: apoelstra