merge revision(s) e626da82, f3af5ae7: [Backport #20311]

	Don't pin named structs defined in Ruby

	[Bug #20311]

	`rb_define_class_under` assumes it's called from C and that the
	reference might be held in a C global variable, so it adds the
	class to the VM root.

	In the case of `Struct.new('Name')` it's wasteful and make
	the struct immortal.

	Make Struct memory leak test faster

	[Bug #20311]

	It times out on some platform, so we can reduce iterations.
	On my machine it completes in 250ms and RSS grows 8X.