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.
