absl::hash -> absl::(flat|node)_hash_(map|set) have a big parsing cost

[MBkkt]

All this from #include "absl/hash/internal/hash.h"

// looks useless
#include <algorithm>
// necessary only for overloads
#include <array>
#include <deque>
#include <list>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <vector>

Needed only if we use some of it as Key.
But in practice it's very rare!

What do you think about create separate header without all this absl/light_hash.h
And provide option to use it in absl/container/(flat|node)hash(map|set).h

Same about strings/cord.h and algorithm/container.h, I made a patch for they:
ad88cb8
f692a0b

I understand that you change all headers is very difficult for you, but maybe at least provide option and new header?