ImageMath: Only compute ops dict when needed

[hugovk]

An idea to fix #7717, only compute the ops dict when needed, when first calling eval, and functools.lru_cache it so it's only computed once.

Also rewrite as a dict comprehension. Timings for non-cached use:

200000 loops, best of 5: 1.87 usec per loop
->
200000 loops, best of 5: 1.53 usec per loop

[radarhere]

I wonder if #7721 is a simpler and faster solution?

[hugovk]

Yes, that's better, thanks!