Add ScarcePolynomial

[yalishanda42]

• Create ScarcePolynomial which should have a memory-efficient implementation for polynomials with fewer terms but larger degrees (e.g. $ x^10000 + x^100 + 1 $).
  • Current problem is that a polynomial is represented as a list of its coefficients (the index is the degree of the corresponding term). A polynomial like x^10000 + x^100 + 1 for example would be represented by a list of length 10001 with 9999 zeroes in it. We can easily see how that is a problem.
• Write unit tests for it