Split and modularize the parser

[netvl]

Currently the parser occupies a single module file. It is really inconvenient since it is has more than thousand lines of code and hence is very unwieldy for navigation and modification. It was written before I knew how to split large implementations across different modules. This has to be refactored.

Basically, I want to split parser implementation into several submodules, each responsible for different part of XML grammar. Each module should provide a method or a couple of methods to parse the corresponding grammar part, and these methods should then be called from the parent module - essentially the same thing I did in md.rs.

This has to be done ASAP as this prevents easy modifications in the parser.