Re. builtin intefaces and naming/namespacing

[ParadoxV5]

[Enumerator::_Each] is not a replacement for _Each
⸺ #1915 (comment)

Namespaced or not, it might be more organizational to dissolve builtin.rbs’s convenience types to more relevant places, such as enumerable.rbs in ::_Each and _ToInt&int in integer.rbs.

I think _MethodName should be restricted to interfaces that implement that method_name minimally.
⸺ #424 (comment)

Would it be worthy to add lower-case interfaces _example?
And start a new convention where _method_name implements #method_name and _ClassName is a duck type.
Examples:

rbs/core/builtin.rbs

Lines 206 to 220 in 45b34bd

	interface _Range[T]
	# The beginning value, `nil` if there is no beginning.
	def begin: () -> T?
	# The ending value, `nil` if there is no ending.
	def end: () -> T?
	# Whether or not to include the end in the range.
	def exclude_end?: () -> bool
	end
	interface _Exception
	def exception: () -> Exception
	| (String arg0) -> Exception
	end

interface _Range[T]
  def begin: () -> T?
  def end: () -> T?
  def exclude_end?: () -> bool
end

interface _exception
  def exception: () -> Exception
               | (String arg0) -> Exception
end