[ruby-core:111306] [Ruby master Feature#19236] Allow to create hashes with a specific capacity from Ruby

["byroot (Jean Boussier)" <noreply@ruby-lang.org>]

Issue #19236 has been reported by byroot (Jean Boussier).

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Feature #19236: Allow to create hashes with a specific capacity from Ruby
https://bugs.ruby-lang.org/issues/19236

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Target version: 3.3
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Followup on [Feature #18683] which added a C-API for this purpose.

Various protocol parsers such as Redis `RESP3` or `msgpack`, have to create hashes, and they know the size in advance.
For efficiency, it would be preferable if they could directly allocate a Hash of the necessary size, so that large hashes wouldn't cause many re-alloccations and re-hash.

`String` and `Array` both already offer similar APIs:

```ruby
String.new(capacity: XXX)
Array.new(XX) / rb_ary_new_capa(long)
```

However there's no such public API for Hashes in Ruby land.

### Proposal

I think `Hash` should have a way to create a new hash with a `capacity` parameter.


The logical signature of `Hash.new(capacity: 1000)` was deemed too incompatible in [Feature #18683].


@Eregon proposed to add `Hash.create(capacity: 1000)`.





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