Add support for namespaces in tuple assignment

Add support for namespaces in tuple assignment

This fixes a bug that existed because namespaces within `{% set %}`
were treated as a special case. This special case had the side-effect
of bypassing the code which allows for tuples to be assigned to.

The solution was to make tuple handling (and by extension, primary token
handling) aware of namespaces so that namespace tokens can be handled
appropriately. This is handled in a backwards-compatible way which
ensures that we do not try to parse namespace tokens when we otherwise
would be expecting to parse out name tokens with attributes.

Namespace instance checks are moved earlier, and deduplicated, so that
all checks are done before the assignment. Otherwise, the check could be
emitted in the middle of the tuple.