Turn Expr into a proper range instead of a view

[Krzmbrzl]

Previously, the Expr implementation used range-v3's view facade to
implement range semantics on Expr objects. However, views are different
things from containers which had some more or less subtle consequences.
See also https://stackoverflow.com/a/31462435

This PR replaces the view facade with virtual begin/end
implementations returning ExprIterator objects. Hence, Expr is now a
fully fledged container that is usable as a random-access range.

[Krzmbrzl]

Okay, I'll have to change the impl to make some protected helper functions like get_begin() & Co which are then called in Expr's base begin()/end() functions to get rid of these ridiculous shadowing issues.