rework wraps_column_expression logic to be purely compile time checking

rework wraps_column_expression logic to be purely compile time checking

Fixed issue where :func:`.select` of a free-standing, unnamed scalar expression that
has a unary operator applied, such as negation, would not apply result
processors to the selected column even though the correct type remains in
place for the unary expression.

This change opened up a typing rabbithole where we were led to also
improve and harden the typing for the Exists element, in particular
in that the Exists now always refers to a ScalarSelect object, and
no longer a SelectStatementGrouping within the _regroup() cases; there
did not seem to be any reason for this inconsistency.

Fixes: #12681
Change-Id: If9131807941030c627ab31ede4ccbd86e44e707f