Break scalars() and mappings() into separate objects

Break scalars() and mappings() into separate objects

The issue of Result.fetchXXX()  methods returning Row
objects unless filtering is applied will not provide a
clear enough API story when type annotations are applied,
so break out scalars/mappings into separate wrapper objects.
this makes some things more intuitive and other things a little
more bumpy.   however the return type story is now clearer.

Fixes: #5503
Change-Id: I629a061823179680dc0723559183859a67ea4db1