Fix memory leak caused by caching

[lephyrus]

Short version: Even though the documentation says otherwise, using bundle as config.cache leaks memory and will probably throw an error in the future. Unfortunately, the Rollup documentation does not (yet) reflect this.

Long version:
We've migrated a somewhat sizable library to a Rollup build and use this preprocessor for testing. When watching for changes, Node crashes after typically 2-3 file changes (FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory). While investigating the problem, I found that not passing a cache fixes the problem (but obviously slows down the build a lot).

I then found this issue, for which a fix was released just days ago. I upgraded to Rollup v0.66.5, but the problem remained. Re-reading the issue, passing a previously generated bundle as the config.cache option of a subsequent bundle is apparently bad. It may work, but it's not the intended usage and introduces a memory leak. The documentation will hopefully be adapted and passing a bundle may throw in the future. Instead, bundle.cache should be passed in config.cache. Doing so solves our crashes and works as intended in terms of caching.

[lephyrus]

@jlmakes May I ask for a timeline on the next release? This would help me decide if a major PR for a project at work should wait for this fix or go with my fork for the time being. Thanks!

[jlmakes]

Thanks for the contributions @lephyrus !!
Check out 6.1.0 (diff: 6.0.1...6.1.0)