API changes to HasReaderOptions, HasHeaderMap, HasIdentifierList.

Previously these were typeclasses of monads.  They've been changed
to be typeclasses of states.  This ismplifies the instance definitions
and provides more flexibility.

This is an API change!  However, it should be backwards compatible
unless you're defining instances of HasReaderOptions, HasHeaderMap,
or HasIdentifierList.  The old getOption function should work as
before (albeit with a more general type).

The function askReaderOption has been removed.
extractReaderOptions has been added.
getOption has been given a default definition.

In HasHeaderMap, extractHeaderMap and updateHeaderMap have been added.
Default definitions have been given for getHeaderMap, putHeaderMap,
and modifyHeaderMap.

In HasIdentifierList, extractIdentifierList and updateIdentifierList
have been added.  Default definitions have been given for
getIdentifierList, putIdentifierList, and modifyIdentifierList.

The ultimate goal here is to allow different parsers to use their
own, tailored parser states (instead of ParserState) while still
using shared functions.