Update Struct

[sampersand]

This PR updates Struct's definitions, as well as adds in the Hash::_Key interface (needed for Struct#deconstruct_keys), which will be used when I work on Hash.

There's one small caveat: There's no real way to annotate <subclass of Struct>.new. After talking it over with @soutaro, we've decided it's fine and users can just add their own news type definitions if needed. e.g.:

Point = Struct.new(:x, :y) do def to_s = "(#{x}, #{y})" end

would become

class Point < Struct[Numeric]
  attr_reader x: Numeric

  attr_reader y: Numeric
  
  def initialize: (Numeric x, Numeric y) -> void
  
  def to_s: () ->  String
end

[soutaro]

After thinking about the type definitions and typical Ruby programs using Struct, having a generic parameter here seems unnecessary to me.

  def self.new: (string? classname, *interned fields, ?keyword_init: boolish?) ?{ (singleton(Struct)) [self: singleton(Struct)] -> void } -> untyped
              | (Symbol field1, *interned fields, ?keyword_init: boolish?) ?{ (singleton(Struct)) [self: singleton(Struct)] -> void } -> untyped

Probably, having singleton(Struct) as generic type upper bound would improve, but it looks like using untyped makes sense here. (And we no longer need _StructClass with the type definition.)

[soutaro]

It looks like the hash.rbs update contains unexpected indentation?

[soutaro]

👍

I will try pushing commits with docs generated by bin/generate_docs and see what happens.