Allow creation of metadata without matching result

Each node in the result can only be matched once in any given mode, and
transforms need a mechanism for inserting elements without requiring a
match. This commit gives them a means of doing so by also matching
every `<xslt:include>` in the main transform. If a transform has a
`书社:id` top‐level element which is an i·r·i, then its include will
have a corresponding attribute, and transforms can (by convention)
match this include without fear of conflicts.

This commit also makes the expansion and result available as top‐level
variables in the `书社:` namespace, so that transforms can easily match
within them.