new std.array.replaceLast

While testing of replaceFirst with utf8 string I found a bug and fixed that.
This lead me to add replaceLast because I needed such a function in the past and didn't want to retro four times.
You could also say that the problem is that arrays are strings and vice versa. Not that I'm saying that, string being arrays is great IMO, but only pointing the finger at autodecoding is a two narrow pov.