Faster String Operations

The UTF-8 encoding has the drawback that characters are not all the same size: they are either one, two, or three bytes in length. This makes operations like string length, string index, and string range quite slow in comparison to a system that uses fixed-sized characters. A new Unicode string type, which uses 16-bit characters, was added to support faster string operations. This does not cause any changes that are visible to Tcl scripts, except for improved performance in comparison to Tcl 8.1.