Changed all inner types to a more peformant (static) structure by default.
This changes the way casts work in that other types can no longer be
auto-casted without copying. To use the old behavior specify -D
HXMATH_USE_DYNAMIC_STRUCTURES. This addresses issue #16 and brings the
time taken by operations on abstracts down to
2x~0.75x that of the hardcoded version in the stress test.