The interface is a bit clunky, the "author" and the callback "data"
have to be specified when adding (an) implementation(s) in order
for this to work. But it does, and this is just a PoC, the interface
could be cleaned up.
Implements a totally simplistic "poortf" mutable typed function and
a picomath instance generator, as well as the very beginnings of a
number type and one generic function and a default full picomath instance.
Also provides some tests which serve as usage examples.
