Rename add_function to add_udf
Rename function non-terminal to udf
Fix comment typo
Rename function invocation rules to not mention udf
s/resolve_udf/resolve_function/g
Merge remote-tracking branch 'origin/shumo_add_string_functions' into andrew_sdf
Add simple unary expressions to expressions_library
Use expression_library for simple unary functions
Do not recursively expand variables during function expansion
Make len a simple unary function
Better comment for simple unary functions
Support binary one-to-one functions
Determine arity auto-magically
Move aggregate functions into expressions_library
Expand COUNTALL in expressions_library.py
Add exception for reserved token
Check for reserved word usage throughput parser
Use import * for myrial exceptions
normalize function name in is_defined
Test that reserved column name generates an exception.

Fixes #181