Arbitrary self types v2: deshadow quicker

A previous commit added a search for certain types of "shadowing"
situation where one method (in an outer smart pointer type, typically)
might hide or shadow the method in the pointee.

Performance investigation showed that the naïve approach is too slow -
this commit speeds it up, while being functionally the same.

This still does not actually cause the deshadowing check to emit any
errors; that comes in a subsequent commit which is where all the tests
live.

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