<rule> may now declare <let name= value=> bindings, evaluated at each
context node in order (so a later binding can reference an earlier one
through $name). The bindings are available to the rule's assert/report
tests and to <value-of select="$name"> in messages, using the XPath
engine's existing variable support.
Schema- and pattern-level let, abstract rules/extends, diagnostics, and
key()/document() remain.
Part of #80
