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.
And make it so that if the same author adds imps again, they replace
the previous imps from that author.
Use this feature to avoid the explicit check for double-inclusion in
subtract, and test it worked.
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.
