Eigen/gmp memory management analysis

[justin1121]

As far as I can tell Eigen is automatically cleaning up its dynamic memory by itself as things go out of scope. We should do some experimentation to make sure this is actually what's happening. Perhaps gmp memory isn't getting cleaned up properly.

[mortendahl]

It might also be interesting to (eventually) figure out when memory can be reused. Some operations in GMP might do that (say, add_assign) when they know that their overwriting.