PERFORMANCE: Add memoization to findGlobal()

[HenrikBengtsson]

By adding support for memoization to lookup of global of functions / closures, we should be able to avoid replicated searches of the same code base.

The tricky part is whether this can be done with a 100% accuracy, e.g. can the set of globals (the names not the values) ever change for a given function (as long as the function itself does not change)? Changes in dependencies, options, par's, expressions, globals of globals, ...