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).
key(name, node-set), including key('k', .), now unions the indexed nodes
for every value node's string value and de-duplicates, instead of using
only the first node's string value. A string value still works directly.
Part of #82.
