WKT lexer

As I found myself some spare time and enthusiasm, I tried to write a lexer to replace the regex-based parsing of WKT strings (see also #2172). Here is the result. Please let me know whether this is anywhere near the concepts that have been conceived.
FYI, a short test on the `ol.format.WKT.GeometryEncoder_` surprisingly shows an **increase** of 22 bytes...