Modify AST to represent function names as refs

These changes update the AST to represent function names as refs.
Previously, function names were represented as strings. Representing the
names as strings was fine, however, once functions and rules are
merged, it will be desirable to refer to functions using references.
This is a bit of preemptive refactoring to make that change easier.
Instead of having functions referred to with both strings and
references, all functions will be referred to with references.