exact/config.h: Fix a typo
SConstruct: Remove unused gmpxx lib from gmp external
exact/perturb: Remove degree limitations by avoiding precomputation

Instead of precomputing the special upper triangular factors in the
vandermonde script, we now factor the upper triangular factor into
a product of special upper bidiagonal matrices with extremely simple
structure.  No more precomputation, and therefore no more degree
limitations.
Use :x() instead of :x(0) in a few constructors
exact/Interval: Use double for Interval computation

This increases the accuracy of filters and constructions, and doesn't
affect memory bandwidth since we never store Intervals.
Add a version of asarray taking a static size C array
exact: Remove obsolete permutation_id routine
Add symbolically perturbed exact circle csg

Circle csg turns out to involve quite large predicates: the current
code uses degree 20, although this is reducible to degree 12 with
no algorithmic changes and degree 8 if ray firing is adjusted to use
rays at integer heights.  In addition, representing nearly straight
lines with circular arcs sacrifices nearly half the quantization bits
in order to place very large radius circles very far away.

For both of these reasons, the ExactInt type has been bumped from
int32_t to int64_t, with quantized values in the range (-2**53,2**25).
This causes a potentially significant slowdown, but correctness wins.