Switch to deterministic pseudorandom symbolic perturbation

No more code generation with exponential blowup!  Unfortunately, there
is currently a two order of magnitude slowdown for the degenerate case
due to memory allocation overhead in GMP and/or GMP's C++ interface.
For example, test_exact.py's "corner delaunay origin" takes 0.2388 s,
vs. 0.0051 s for nondegenerate input.

More pleasantly, ./test_exact.py -b slows down by only 2.3093 s to 2.4160 s,
small enough to not care (and this may also improve due to optimizations
for the degenerate case).
exact: Avoid GMP's C++ API

Using the C++ API causes significant unnecessary memory allocation.
Using the base C API instead speeds up ./test_exact.py's "corner
delaunay origin" from 0.2388 s to 0.0808 s.