perf: cache command locations

[TorchedSammy]

this means that hilbish can save time searching $PATH for an executable. if $PATH ever changes, the cache is invalidated. this is similar to how bash functions and also provides the hash command.

should the command be named hash though? it's not immediately obvious, and posix compat isnt a concern so it could just be called ccache or anything else that fits. it would also be less confusing as a lua function, since it of course needs to be exposed to lua too.