Group @testable imports at the bottom, per public/rules-swift's swiftformat. A
behavior-preserving import-ordering reformat across 252 files (mostly tests).
--trailingclosures was deliberately not adopted (cosmetic, and swiftformat flags
it as caution-worthy).
The RNC datatype parameter block xsd:int { minInclusive = "5" } is now
parsed into facets (previously skipped), matching the XML-syntax <param>
support. The facet mapping is shared by both parsers through a new
RelaxNGFacets helper.
Part of #79
