Enforce boolean result type for all eq_, is_, isnot, comparison

Enforce boolean result type for all eq_, is_, isnot, comparison

Repaired issue where the type of an expression that used
:meth:`.ColumnOperators.is_` or similar would not be a "boolean" type,
instead the type would be "nulltype", as well as when using custom
comparison operators against an untyped expression.   This typing can
impact how the expression behaves in larger contexts as well as
in result-row-handling.

Change-Id: Ib810ff686de500d8db26ae35a51005fab29603b6
Fixes: #3873