function last( str, n ) {
	var len = str.length;
	return str.substring( len - n, len );
}
Returns the last `n` UTF-16 code units of a string.

@param {string} str - input string
@param {NonNegativeInteger} n - number of UTF-16 code units to return
@returns {string} output string

@example
var s = last( 'hello world', 1 );
// returns 'd'

@example
var s = last( 'this is stdlib', 1 );
// returns 'b'

@example
var out = last( 'JavaScript', 6 );
// returns 'Script'

@example
var out = last( 'New', 6 );
// returns 'New'