libexpr: don't allocate additional set in builtins.listToAttrs

[NaN-git]

Motivation

At the moment this function uses an additional std::set to filter duplicate keys. This causes many additional allocations for the nodes in the set.
Finally the Bindings are sorted, although the set contains the keys in sorted order already.

Context

The additional std::set is not necessary. Instead the Bindings are sorted and filtered in a single array.

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[xokdvium]

Optimization LGTM. It's a bit hacky, since it slightly abuses the fact Bindings::operator[] does not check that the access isn't out of bounds of size_. In the future we might want a cleaner API for doing such optimizations.

[NaN-git]

Optimization LGTM. It's a bit hacky, since it slightly abuses the fact Bindings::operator[] does not check that the access isn't out of bounds of size_. In the future we might want a cleaner API for doing such optimizations.

In C++ the expectation is that operator[] doesn't do any bounds checks. The convention is to use at(size_t) instead.

[roberth]

Thank you @NaN-git!