Add concept of "implicit boolean", treat as native

Add concept of "implicit boolean", treat as native

Fixed issue that is closely related to :ticket:`3639` where an expression
rendered in a boolean context on a non-native boolean backend would
be compared to 1/0 even though it is already an implcitly boolean
expression, when :meth:`.ColumnElement.self_group` were used.  While this
does not affect the user-friendly backends (MySQL, SQLite) it was not
handled by Oracle (and possibly SQL Server).   Whether or not the
expression is implicitly boolean on any database is now determined
up front as an additional check to not generate the integer comparison
within the compliation of the statement.

Fixes: #4320
Change-Id: Iae0a65e5c01bd576e64733c3651e1e1a1a1b240c