Improve caching of equivalent variables for Analyser

[hsorby]

Whilst reviewing PR #1366, I played around with alternatives for caching equivalent variables.
I found that I could reduce memory usage and improve speed by changing the data structure for caching equivalent variables.

[agarny]

Do you have some metrics to share?

[hsorby]

For the big model test the cache drops from ~7.5 million entries to ~5K entries. The time taken to run the test is halved. For smaller tests the difference is negligible.

[agarny]

Ok, nice!