- Fixed issue where a straight SELECT EXISTS query would fail to

- Fixed issue where a straight SELECT EXISTS query would fail to
assign the proper result type of Boolean to the result mapping, and
instead would leak column types from within the query into the
result map.  This issue exists in 0.9 and earlier as well, however
has less of an impact in those versions.  In 1.0, due to #918
this becomes a regression in that we now rely upon the result mapping
to be very accurate, else we can assign result-type processors to
the wrong column.   In all versions, this issue also has the effect
that a simple EXISTS will not apply the Boolean type handler, leading
to simple 1/0 values for backends without native boolean instead of
True/False.   The fix includes that an EXISTS columns argument
will be anon-labeled like other column expressions; a similar fix is
implemented for pure-boolean expressions like ``not_(True())``.
fixes #3372