Remove unnnecessary postfix increment operation.

As per the C++11 spec, ranged-based for statements only require prefix
increment operators. Given that we don't want to support mutable
iterators as a rule, and supporting both operators increases complexity
and the potential for error, we're just going to drop postfix operators
from our API spec.

If we expand our "functional" APIs to support more general map/fold/etc
operations, we'll almost certainly adopt an immutable API set that does
not depend on ranged-based for compatibility.

Issue: PLCR-527
Stub out a skeleton disjoint union/monadic result class.

Issue: PLCR-527