Add Disk cache

[porcuquine]

Add CACHE types:

• MEM-CACHE
• DISK-CACHE
• DISK-BACKED-MEM-CACHE

If ORIENT_CACHE_DIR is set, the named directory is used for DISK-BACKED-MEM-CACHE cache files when invoking from CLI (including web option). Otherwise, MEM-CACHE is used.

The cache key depends on both system and input, but does not normalize these for order.

TODO:

• Add option to invoke with no cache.
• Limit size of mem cache.