Document how mustachio's AOT compiler works

[srawlins]

This documentation has been a long-time coming. I have an improvement to the deduplication code. But in order to make any sense of what the current deduplication code does, why it could be better, and what the improvement does, I need to add these docs, which describe how the AOT compiler works in general, and particularly how the partial renderer functions are written and optimized.

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[srawlins]

cc @dart-lang/analyzer-team if you want to review this, keep in mind that I'd request you to be the same reviewer for an upcoming PR to improve the deduplication strategy in the mustachio AOT compiler.

[keertip]

Great job on the documentation! Very detailed!

[srawlins]

Thanks!