Expose Warpable::warpSpeed to Squirrel scripts
Fix a comment in StarEnum constructor
Prevent StarCaches from growing limitlessly

The galaxy contains finite but very large number of sectors. A sector
is currently 15 ly^3, which means there are about 4.0e5 sectors in the
galaxy.  Although the player won't have time to look at all of them,
it's no use remembering every cached sector.  We will forget about
old cached sectors after 1000 sectors are remembered, excluding sectors
containing materialized solar systems.  Forgetting materialized sectors
cause the solar system to re-materialize when the player warps to there.

The logic to determine the oldest sector could be improved to use last
accessed order instead of creation order.