[relax-ng] Simple type assignment

I spent some time last week talking to developers at a major XML vendor.
One interesting bit of feedback was in relation to simple type assignment,
by which I mean assigning to each text node and attribute value in the
instance either a single simple type (in the non list case) or a sequence of
simple types (in the list case).  I asked whether this was a useful variety
of type assignment and the response was an unqualified and unanimous yes.
(This isn't saying other varieties aren't useful, just that this variety is
useful.)

I think  we should provide this as one option in any type assignment spec,
especially as we already have a way to do this (it's essentially what we
used for the key/keyref facility that we eventually got rid of).

James