add FunctionElement subclassable Function-like object; document standard "subclass" targets in ext.compiler

The most common use for compiler will be to subclass functions and other column expressions. Base Function off of FunctionElement. Add a guide to the compiler documentation illustrating examples for creating common subclasses, all based on an XXXElement subclass - meaning, ColumnElement, DDLElement, FunctionElement, and perhaps a FromElement, though that one is more involved as far as getting column collections and such.

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