Function names don't need to be keywords

The JPQL spec specifies that function names are reserved words. This *sorta* makes sense, since some functions like `cast`, `extract`, and `trim` have an irregular syntax inherited from SQL. But even given if we need to say that `cast`, `extract`, and `trim` are keywords (and even that's probably not necessary), we don't need to say that the names of all the regular functions are.

The issue names no repository file or test. Start by reading the JPQL specification's rules for function names and comparing the treatment of regular functions with cast, extract, and trim. Done requires a decided specification change that clearly identifies which names remain reserved.

Written by the indexing model from the issue text.