PropValue now knows its PType

We're not doing very much with that yet, but it should allow us to merge
PropValue and TypedValue, finally.
PropValue.render() no longer requires an elemT

Since it now knows about that itself.

Found and fixed a bug in ElemValue along the way.
Removed TypedValue's unneeded PType

Checkpoint towards merging TypedValue and PropValue.
Removed the TypedValue.cut parameter

Checkpoint
Began blending TypedValue and PropValue

Checkpoint, but we're starting to get there.  Made TypedValue simply an
ancestral trait to PropValue, with little content of its own.  This lets
me eliminate a *lot* of creation code, because the TypedValue is no
longer wrapping the PropValue, it *is* the PropValue.
PropValue is now vestigial

Moved the entire contents of PropValue into TypedValue.

Also, began rationalizing some of the names of those contents.
Eliminated PropValue

In favor of TypedValue.  Yay!
Renamed TypedValue to QValue

Nearing the end of The Great Value Refactoring.
Simplified PropAndVal

And along the way, removed a lot of cruft from Property that now clearly
doesn't belong there.