Added Grounder::Split(), which caches split setups.

The whole purpose of Grounder::Split() is the caching. The cache is
part of Grounder because it is depends on Grounder::Ground()'s setup:
when Grounder::Ground() re-grounds, the cache must be emptied to avoid
false lookups.

We do not update the cache when new clauses without new names are
added, as this should have the same cost (in terms of
Setup::AddClause()) as spawning new children later.

All experiments so far are strongly in favour of a cache. Minesweeper
is about twice as fast; test-siblings.lela is >25 times as fast.

An analogous cache for assignments will be added later.