Add it-then-else operator to Booleanizer

[cgjohannsen]

An if-then-else operator takes in three arguments: one bool and two of any other type (so long as they have the same type). We'll use the standard ?: C-style syntax.

For example, this would allow us to have the following C2PO file:

INPUT
b0: bool;
i0, i1: int;

FTSPEC
(b0 ? i0 : i1) > 0: