memoize the generation of MacrosConfig cache keys per (project, appRoot)

[raycohen]

Ref #1399

This is the most aggressive memoization I can think of, computing the cacheKey only once per (project, appRoot) "tuple".

This drops the time spent in babelPluginConfig from about 3 seconds (speedscope in linked ticket) down to about 200ms

[NullVoxPopuli]

What does performance look like in your app with this change?

[SergeAstapov]

just a small note, seems related to #1102, not sure if optimisation made there was lost (we don't see that issue, although we may not be using the latest versions yet)

[raycohen]

@SergeAstapov Thanks for the call out. That memoization is still in place from what I can tell.

@NullVoxPopuli Updated description with a speedscope image - this dropped time spent here to about 200ms (down from ~3s)