[relax-ng] list//interleave

The restriction we agreed on interleave isn't sufficient for the case of
interleave inside list.  Exponential blowup is possible using
list/interleave/value.

One solution is

1. Disallow list//interleave//data
2. Require all <value> in a given interleave to have the same type and
datatypeLibrary.
3. Make it an error if there is a value that occurs in both operands of an
interleave

Another possible solution would be to disallow interleave in list altogther.

James