Add the caching mechanism already present for cli arguments to the
evaluation loop.

This means that an expression of the form `(builtins.getFlake
foo).bar.baz` will be cached if it can be

This is a dirty hack to make caching temporarily work.

The issue seems to be that the cache object is never deleted, which in
turn means that we never commit to the db (as it happens in the object's
destructor), so the cache doesn't persist between two Nix calls.
So we manually commit to the db each time. This is both slower and ugly,
but at least it works for the moment

The others don't need it and it was taking some useless space in the
`Value` type