remove support for 'permissive' parsing

where permissive means that unknown identifiers didn't causing parsing
to abort with an error, but just be flagged as _unknownidentifier.

this was only used in an obscure and little-used part of the GUI,
namely input boxes where the input type was "command" or "reporter".
it's conceivable it might have proved useful for something else
someday, but that's a bridge we can cross when we come to it, if
we ever come to it.  at the moment, it's just getting in the way
as I refactor.
code improvement in IdentifierParser

don't use two maps where one would do
make IdentifierParser return an Iterator not a Seq

it had no particular reason to be strict that I could see,
so why not just let it be lazy