ast: Improve set implementation to memoize groudness

Previously sets were not memoizing the groundness check so each call
to `IsGround` would iterate until it found a non-ground term. In the
worst-case, this would check the entire set. Because we call
`IsGround` as part of the inner-loop of the
`topdown.evalTerm#enumerate` code-path for sets, this introduced a
cross-product!

This commit improves the set implementation to maintain a bit that
indicates whether the set is ground. When new elements are added to
the set, the bit is set accordingly.

This change provides a significant speedup for large sets.

name                   old time/op  new time/op  delta
SetIteration/10-20     28.6µs ± 1%  27.3µs ± 1%   -4.65%  (p=0.008 n=5+5)
SetIteration/100-20     263µs ± 1%   152µs ± 1%  -42.20%  (p=0.008 n=5+5)
SetIteration/1000-20   12.5ms ± 0%   1.4ms ± 1%  -88.67%  (p=0.008 n=5+5)
SetIteration/10000-20   1.12s ± 0%   0.02s ± 1%  -98.63%  (p=0.008 n=5+5)

Signed-off-by: Torin Sandall <torinsandall@gmail.com>