Cache the result of the compile_expr function

[mdickinson]

This PR adds an LRU cache for the compile_expr function, to eliminate unnecessary calls to _as_graph. I also made a one-line change in the observe function so that all expression compilation goes through compile_expr.

I've also boosted the size of the three parsing-related caches from 128 entries to 256.

I opted not to do a per-Expression-object caching of the result of the _as_graphs call (as suggested in the #1516 discussion); it adds extra complication in that it makes the Expression objects mutable, and so introduces potential thread-safety concerns; in contrast, lru_cache has thread-safety provisions built in. There's benefit in keeping the Expression objects as simple immutable pure data structures - I'd rather not change that unless there's clear evidence that it's beneficial performance-wise.

No test changes or additions, because there are no observable behaviour changes.

Closes #1510

See also #1516, #1517