Inspired by https://github.com/josdejong/mathjs/discussions/2212 and
https://github.com/josdejong/mathjs/issues/2585.
Provides a simple adapter function `adapted` which takes a class
implementing an arithmetical datatype and returns a PocomathInstance
with a new type for that class, invoking the methods of the class
in a standard way for the Pocomath/mathjs operations. (That instance
can then be installed in another to add the new type to any instance
you like, including the default one.)
Uses this facility to bring fraction.js Fraction into Pocomath, and
tests the resulting type.
Currently the "standard" interface for an arithmetical type is heavily modeled
after the design of fraction.js, but with experience with other 3rd-party types
it could be streamlined to something pretty generic (and the Fraction
adaptation could be patched to conform to the resulting "standard"). Or a
proposal along the lines of https://github.com/josdejong/mathjs/discussions/2212
could be adopted, and a shim could be added to fraction.js to conform to
**that** standard.
Resolves#30.
