fix: ensure that tail-recursive List.flatten is used everywhere

[TwoFX]

This PR marks List.flatten, List.flatMap, List.intercalate as noncomputable to ensure that their csimp variants are used everywhere.

We also mark List.flatMapM as noncomputable and provide a tail-recursive implementation, and mark List.utf8Encode as noncomputable, which only exists for specification purposes anyway (at this point).

Closes #12676.

[mathlib-lean-pr-testing]

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[leanprover-bot]

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