Rework documentation for parser combinator.

Change name from 'Items' to 'Parser'
====================================
This one was a long tim comming. Items was a transient name due to the
parser combinator library willing itself into existance without my
consent.

Made a bunch of classes private
===============================
The ParseDirective classes only present to enable a "primitive" global
parser are now marked private. There is zero reason to actually
instanciate them more than once, but they still need to be classes due
to the API. The corresponding global "variables" now have attached
documentation and get exported by sphinx.

Distribute documentation
========================
Previously almost all documentation for the parser combinators lived in
the header of the file, but is now distributed onto the declarations.
The previous version was mostly due to me not understanding how to
export global "variables" to sphinx.