class constructor

Extension API: support for lazy emplace.

Looks up key in the table. If found, returns the iterator to the element.

Otherwise calls `f` with one argument of type `raw_hash_set::constructor`,

and returns an iterator to the new element.

`f` must abide by several restrictions:

- it MUST call `raw_hash_set::constructor` with arguments as if a

`raw_hash_set::value_type` is constructed,

- it MUST NOT access the container before the call to

`raw_hash_set::constructor`, and

- it MUST NOT erase the lazily emplaced element.

Doing any of these is undefined behavior.

For example:

std::unordered_set

<ArenaString

> s;

// Makes ArenaStr even if "abc" is in the map.

s.insert(ArenaString(

&arena

, "abc"));

flat_hash_set

<ArenaStr

> s;

// Makes ArenaStr only if "abc" is not in the map.

s.lazy_emplace("abc", [

&

](const constructor

&

ctor) {

ctor(

&arena

, "abc");

});

WARNING: This API is currently experimental. If there is a way to implement

the same thing with the rest of the API, prefer that.