Cache sorted variables to improve compile performance

[chrispcampbell]

I noticed while working on #527 that calling the xxxVars functions (e.g., auxVars) results in some redundant sorting work in the case where sde generate is used with both --list and --outformat (JSON listing in addition to code gen). This is the case for any project using the sde bundle or sde dev commands. The reason that the work is redundant is that these functions are called once during code gen, and then again when generating the JSON listing.

By caching the return value of these functions in a Map, the execution time for sde generate --outformat=c --list for the En-ROADS model goes from 5.2s to 4.9s (on my M1 Max), about a 5% improvement.