Adds:
- ast/UserFunction AST node for user-defined functions.
- astbuilder/AvailableUserFunction: reference to a parse tree for a user function.
- The AST builder only generates ASTs for user functions that are actually referenced.
- astbuilder/UserFunctionVisitor: visits (builds an AST for) a parse tree for a user function.
- This happens after the .src or .inc file has been otherwise processed, so this class serves to hook up the correct SourceFileIdentifier to the AST.
