exact: Switch back to storing quantized values as doubles

Now that we're using 64 bits, interval arithmetic will be both
more important and more useful as a filter.  Storing values as
doubles avoids a conversion before it can be applied.
exact: Avoid GMP memory allocation overhead via the low level interface

Instead of using mpz_t, and occasionally mpq_t, we now perform all
computations on RawArray<mp_limb_t> using the low level interface of
GMP.  This moves nearly all memory allocation onto the stack, most of it
known at compile time (Exact<d> is now pod, for instance).  The rational
arithmetic used in levels 2 and up has been eliminated, which is easy
since all intermediate results in the divided difference algorithm are
automatically integral.

These changes speed up "py.test -sk delaunay_corner" on 2000 points from
0.0493 s and 0.2353 s (random and degenerate) to 0.0151 s and 0.0722 s.
"time py.test -sk circle" speeds up from 1.126 s to 0.566s.

Profile results now show most of the time spent in actual computation,
rather than memory allocation, so further speedups in the degenerate
path are likely to be difficult.
Merge remote-tracking branch 'origin/exact' into exact