Implement a class for allowing combined string / number variables.

[philippeitis]

This is not a full implementation, but more of a starting point that supports const * (collection of strings). It would need to implement add the possible numerical operations before being merged.

This is intended to allow something like:

Polynomial(1, 2) * Polynomial('a', 'b')
>>> ax^2 + (2a + b)x + 2b

and hopefully:

a = Polynomial(1, 2) * Polynomial('a', 'b')
# To differentiable between a.b = 5 and a['b'] = 5.
a['b'] = 5
a
>>> ax^2 + (a + 10)x + 10

[pep8speaks]

Hello @philippeitis! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file polynomial/core.py:

Line 702:1: E302 expected 2 blank lines, found 1
Line 731:17: E131 continuation line unaligned for hanging indent