Get return type statically

[Smoke Adams via Digitalmars-d-learn]

I should point out also that this should be inheritable.

Eventually I would like to create an algebra of SuperFunctions.

e.g., SF3 = SF1 + SF2

is a new super function that combines the parameter list of SF1 
and SF2 and unionizes their return type. Both functions are 
called by Do(which will ultimately be handled by opCall).

Other operations on the parameters can be created(intersection or 
subtraction, multiplication, etc...).

I believe, to do this, I will have to create a string mixin that 
formulates Do(...) properly using a CTFE.