libexpr: Include derivation names in the call stack profile

[xokdvium]

Motivation

This makes the profiler much more useful by actually distiguishing
different derivations being evaluated. This does make the implementation
a bit more convoluted, but I think it's worth it.

Example evaluation of `flutter`

$ nix eval nixpkgs#flutter --no-eval-cache --eval-profiler flamegraph

Context

Follow-up to #13220.
Current output is useful, but hard to interpret for end-users NixOS/nixpkgs#320528 (comment)

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[roberth]

This seems very useful.

does make the implementation a bit more convoluted

Maybe a nicer way to frame and implement this, is as a general way to add data to traces.
We'd have at least these two instances that could be unified in such a framework

• derivationStrict
• addErrorContext

and potentially more, e.g. a thunk type that remembers the mapAttrs attribute name that get computed would also provide valuable context in both error traces and profiling traces.

Doesn't have to all be done in this PR of course, but I feel that by developing this functionality more, it becomes less convoluted.

[xokdvium]

Maybe a nicer way to frame and implement this, is as a general way to add data to traces.

That could be done by probably just making FrameInfo just a polymorphic base class. I initially avoided that to skip allocations and going through virtual functions, but that should probably be fine.

Generally I'm not too keen on forcing arbitrary Values and adding that to traces, because that would skew with the thunk evaluation. The profiler should be as non-intrusive as possible, but the special case of derivations is too important.