merge revision(s) 254bed30,fad3023e,3156fb0f: [Backport #17218]

	Renamed `nurat_sub` compliant with `rb_rational_plus`


	Fix ArithmeticSequence#last and ArithmeticSequence#each for
	non-integer sequences (#3870)

	[Bug #17218]
	[ruby-core:100312]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67936 b2dd03c8-39d4-4d8f-98ff-823fe69b080e