Fix slow pprof for Alloc profiles: Use integer keys; stop stringifying StackFrames (it's expensive)

[NHDaly]

In #70, we fixed the issue in #69 (multiple stackframes hashing to the same thing) by stringifying the stack frame. This turns out to be terribly, terribly slow.

This PR instead fixes the issue by manually constructing a unique Location for each frame in the stack, by pairing the unique function ID with the line number of the frame. This is what is meant by a location: a function + a line number, so that should be a perfectly suitable unique location key.

I also added a rigorous regression-testing file, which compares the profiles produced by any branch to the profiles produced on main. This helped me be confident in this change, and should help us prevent any issues in the future.

[codecov-commenter]

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