This is a breaking interface change: Filter now returns templated
TInterface<TFilter<F,P>> objects instead of TInterface<TFilterBase>
as before (and anagolously for Define and Range).
Jitted transformations still return TInterfaces wrapping the base
classes (e.g. TFilterBase).

Filter, Define and Range returned TInterfaces wrapping pointers to the
base classes representing the corresponding nodes. This choice was made
to limit compile time and ease the introduction of jitted transformations.
As a consequence, nodes of the functional graph communicated with each
other through virtual calls.

This commit lets nodes call into each other through the derived (template)
types instead. Given that all nodes' virtual methods are marked `final`,
this should allow compilers to de-virtualize such calls.