Optimized expressions are not cached

By default `JMESPath#search` caches parsed expressions in order to improve performance. However, it also takes care to call `#optimize` on each expression. The optimized expressions are not cached, meaning that for every `#search` the optimization takes place.

According to my (limited) tests, caching the optimized expressions increases the performance by 10-30 times.

I see two ways to solve this;
- Cache the optimized expressions within `Runtime`, instead of caching the non-optimized expressions in the `CachingParser`, or
- Make `#optimize` memoize its result, so that sub-sequent calls can return the value directly without the overhead

I'll happily help with the coding, but wanted to get some feedback first.