CIF/BDD: support limited forms of internal user-defined functions

I think it wouldn't be to hard to support some internal user-defined functions, with the following concepts:

• Parameters.
• Local variables.
• Assignments.
• if statements.
• return statements.

The following features would not be supported:

• while statements (including break and continue statements).
• Multiple return types/values (already implied by not supporting tuple types).
• Direct or indirect recursive function calls.
• Functions as data.