Add type restrictions to json

[Vici37]

This is the output of compiling cr-source-typer and running it with the below incantation:

CRYSTAL_PATH="./src" CRYSTAL_HAS_WRAPPER=1 ./typify spec/std_spec.cr
--error-trace --exclude src/crystal/ 
--stats --progress 
--union-size-threshold 2
--ignore-private-defs 
--ignore-protected-defs  
src/json

This is related to #15682 .

[Vici37]

I couldn't think of a reason for subkeys to be anything other than ints and strings in a JSON object, so I left the resolved type restriction. Let me know if this is too restrictive for some reason.

[straight-shoota]

Sounds good 👍
But for polish, we should use the same ordering of the union in both places.

Suggested change

	def dig?(index_or_key : String | Int, *subkeys : Int | String) : JSON::Any?
	def dig?(index_or_key : Int | String, *subkeys : Int | String) : JSON::Any?

Ditto below.

[straight-shoota]

suggestion: For retrieval, the type restriction to Int | String makes sense. But here it shouldn't be stricter than that of #string.

Suggested change

	def field(name : String | Int32, value : _) : Nil
	def field(name : _, value : _) : Nil

[straight-shoota]

Sounds good 👍
But for polish, we should use the same ordering of the union in both places.

Suggested change

	def dig?(index_or_key : String | Int, *subkeys : Int | String) : JSON::Any?
	def dig?(index_or_key : Int | String, *subkeys : Int | String) : JSON::Any?

Ditto below.

[straight-shoota]

suggestion: This should probably be Nil like in IO implementations.

[straight-shoota]

issue: This is a constructor, so the return type should be self.
Ditto all below.