As per #810 & #796, only LUA_OPTIMIZE_MEMORY=2 & MIN_OPT_LEVEL=2 are
supported when building. This commit effects that limitation.
With this change modules/auxmods.h no longer needs to be updated for
every new module, nor do module writers need to cater for a hypothetical
LUA_OPTIMIZE_MEMORY < 2 scenario.
Also made the cache on/off configurable via user_config.h. Uncached writes
are not a very good idea, but for read-only deployments a further ~0.5k RAM
can be gained by disabling the cache.
Tweaked the file.read() workhorse to read large chunks at a time rather
than use getc(), to compensate for potential unavailability of cache.