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.
This could be `@private` I think (and named `ol.format.WKT.Lexer_`).  Also minor and not necessary to change.