Use unordered_dense for hash tables

[ajtribick]

The standard library unordered_map has various issues that make it slow. Might be worth investigating https://github.com/martinus/unordered_dense as a replacement.

(It may also be worth considering Boost flat_map for the case where we actually need a sorted map - e.g. for prefix searching in the name manager)

[375gnu]

We don't use unordered_map in any performance critical code afaik.

[ajtribick]

The other advantage of unordered_dense is that it offers the possibility for heterogeneous lookups (e.g. use a string_view to do lookups in a hash table with string keys), which isn't offered by std::unordered_map until C++20.