merge revision(s) bf1a6771,c1463625: [Backport #17012] [Backport #17014]

	Fix non-numeric exclusive Range#minmax bug

	The implementation of Range#minmax added in d5c60214 causes the
	following incorrect behaviour:

	  ('a'...'c').minmax => ["a", ["a", "b"]]

	instead of

	  ('a'...'c').minmax => ["a", "b"]

	This is because the C implementation of Range#minmax (range_minmax)
	directly delegates to the C implementation of Range#min (range_min) and
	Range#max (range_max), without changing the execution context.

	Range#max's C implementation (range_max), when given a non-numeric
	exclusive range, delegates to super, which is meant to call
	Enumerable#max. However, because range_max is called directly by
	range_minmax, super calls Enumerable#minmax instead, causing the
	incorrect nesting.

	Perhaps it is possible to change the execution context in an optimized
	manner, but the simplest solution seems to be to just explicitly
	delegate from Range#minmax to Range#min and Range#max.

	Use static variables in Range#minmax