[Hash] Update Hash's signatures and tests to be correct

[sampersand]

PHEW! This was a long one, but now Hash is finally done!

This PR updates all of the definitions within hash.rbs to be more correct, as well as modernizes the entire Hash_test.rb file, which was still using the old format.

A complete list of functional changes is below. Bolded ones are important.

• Hash::_Pair[K, V] is added; Hash.[] and Hash#to_h use it.
• Hash::_Equals is added. Used by Hash#{assoc,has_value?,rassoc}.
• Hash.[] has been changed to return instances in the Hash[a: 1] and Hash[[:a, 1]] cases, and now uses _Pair
• Hash.try_convert returns nil in the untyped case.
• Hash#{<,>,<=,>=} all now accept hash[untyped, untyped]
• Hash#[] now accepts Hash::_Key
• Hash#any?'s signature has been fixed: the block form actually is a tuple, and now uses Enumerable::_Pattern
  • It would be nice to eventually change Enumerable::_Pattern to take a generic
• Hash#{assoc,rassoc,has_value?,key} now all take Hash::_Equals
• Hash#compact returns an instance. There's no real good way to represent the return value for the function, but instance is more accurate than Hash[K, V]
• Hash#deconstruct_keys now has an untyped argument, as it's ignored
• Hash#{default,dig,delete,except,fetch,fetch_values,has_key?,slice,values_at} now all accept Hash::_Key, and any related blocks also do as well
• Hash#default_proc now returns a (^(self, _Key) -> V)?. I'm not sure if this self is legal, or needs to be an instance (or even Hash[K, V], though I don't like that because it doesnt account for Hash subclasses)
• Hash#default_proc= is similarly plagued with Hash#default_proc's concern. Also, the three branches were split up.
• Hash#each{,_key,_value}'s blocks now returns void not untyped
• Hash#flatten now explicitly handles a level of 0, and now accepts ints
• Hash#{merge,merge!} now both accept hash. Unfortunately, for merge return type is Hash, but really it should be "instance[A | K, B | V]"
• Hash#freeze has been added
• Hash#replace now accepts hash
• The alias pairs size/length, filter/select, and filter!/select! have been swapped, to more closely match documentation. (Other aliases which matched the documentation haven't been touched)
• Hash#to_h's block form can now return _Pair[A, B]
• Hash#{transform_keys{,!}} variants with hash arguments ({a: 1}.transform_keys(a: :b) #=> {b: 1}) have been added
• Hash#initialize's proc now accepts an instance

[sampersand]

Pushed a new version which retains %a{implicitly-returns-nil} on []. removing that should be another PR.