Add `BigInteger` type

[castarco]

Decimalisn't enough to handle integer properties because the following reasons:

• Integer needs encapsulating integer-specific algorithms (like maximum common divisor, or primes factorization).
• We can't continue adding code to Decimal because it's too complex now.
• Decimal is slow because we can't use GMP. On the other hand, using GMP with Integers is a good idea.
• We need a BigInteger type to implement the BigRational type (#27).