feat: parallel execution of validations

[adamhjk]

This PR runs all validations in parallel, rather than in serial. The result is that initial validation runs feel pretty much instantaneous.

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[wendybujalski]

Do I assume correctly that this change means that we're sure all validations are independent of each other and can run concurrently? Could there possibly be a case where two validations would affect each other when running in parallel?

[fnichol]

Whoopsy, we addressed this (finally) in #7450, closing!