gmpy2 support

Currently, PyCall uses strings as the interchange mechanism for `BigFloat` and `BigInt`. This is suboptimal from a performance perspective and also potentially lossy in the case of `BigFloat`.

However, the `gmpy2` module for Python provides a Python interface to GNU MP/MPFR numbers (the same as Julia), and it should be possible (with suitable digging) to pass Julia bignums to Python in binary form (and potentially without making a copy). (Furthermore, when the `gmpy2` module is available in Python, the `mpmath` module automatically uses it anyway, so we might as well take advantage of it for `BigFloat` conversions if it is present.)

I'm not sure if anyone actually cares about better bignum interoperability with Python, though?