* Use a hash table to index existing vtables

This allows for quick deduplication even in situations where there
might be thousands of vtables due to 'combinatoric explosion'.

This fixes issue #5301.

* Refactor 0-offset trimming

* Improve deduplication benchmark

The routine now generates a set of realistic logical layouts and
uses a timer function that randomly picks a layout for each iteration.

The benchmark runs in batches of # of logical layouts = 1, 10, 100, 1000.

(Note that due to alignment, the actual number of vtables is usually slightly
higher.)