issue #38 - implicit p:inline

apply changes to new doc structure ... as per last WG meeting allow exclude-inline-prefixes and expand-text to all elements that can contain p:inline. Allow only a single foreign element. Amend schema.

Diff version of (my) spec
http://xquery.github.io/xproc-vnext-spec/langspec/xproc20/head/xproc20/diff.html