Get a NSString from self with the specified length

Property definitions

cocoa :: foundation $ CString :: to_nsstring
	# Get a `NSString` from `self` with the specified `length`
	fun to_nsstring(length: Int): NSString in "ObjC" `{
		return [[NSString alloc] initWithBytes:self
			length:length
			encoding:NSUTF8StringEncoding];
	`}
lib/cocoa/foundation.nit:44,2--49,3