Support elements in place of expression-containing attributes

[AndrewSales]

My interest in this was revived by this post about XProc and a means to avoid attribute value normalization.

See also: Schematron/schematron-enhancement-proposals#41 and Schematron/schematron-enhancement-proposals#22 (comment).

So for example you could do:

<assert>
<test>
/here/is
  [lots]
  /of-formatting/(
    I-Prefer | to_keep
  )
</test>
</assert>