Prototype API for log redaction

Created by: varungandhi-src

Implements what I describe in my comment earlier.

The PR is written for commit-by-commit review.

We don't need to merge this if we think that this is the wrong direction to go in, but I figured it'd be easier to understand the proposed changes with actual code rather than without.

(I'm not attached to the specific names of the different types, cases etc; feel free to suggest alternate names.)

Test plan

Added tests for non-trivial new functions.