Don't stringify unnamed column elements when proxying

Don't stringify unnamed column elements when proxying

Repaired and solidified issues regarding custom functions and other
arbitrary expression constructs which within SQLAlchemy's column labeling
mechanics would seek to use ``str(obj)`` to get a string representation to
use as an anonymous column name in the ``.c`` collection of a subquery.
This is a very legacy behavior that performs poorly and leads to lots of
issues, so has been revised to no longer perform any compilation by
establishing specific methods on :class:`.FunctionElement` to handle this
case, as SQL functions are the only use case that it came into play. An
effect of this behavior is that an unlabeled column expression with no
derivable name will be given an arbitrary label starting with the prefix
``"_no_label"`` in the ``.c`` collection of a subquery; these were
previously being represented either as the generic stringification of that
expression, or as an internal symbol.

This change seeks to make the concept of "anon name" more private
and renames anon_label and anon_key_label to _anon_name_label
and _anon_key_label.   There's no end-user utility to these accessors
and we need to be able to reorganize these as well.

Fixes: #6256
Change-Id: Ie63c86b20ca45873affea78500388da94cf8bf94