Permit disabling of A* path cache at compile-time

Mostly to measure its effectiveness.  Compile with AS_NO_PATH_CACHE
defined to disable it.

Turns out the path cache is quite effective.  For my continental test
case (Hvy Metal 2 updates), it reduces the number of searches by a
factor of 18.5, speeding up distribution path assembly by a factor of
7.  The price is memory: it uses 135 times more memory than the A*
library.  For my island test case (Hvy Plastic 2 updates), I get 4
times search reduction, 3.5 times faster distribution path assembly,
36 times more memory.