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).
Add the EXSLT common/math/sets modules, dispatched by resolving a function's
prefix to its namespace (FunctionTable.call falls back to namespace lookup),
so any prefix bound to an EXSLT namespace works. math:min/max/highest/lowest/
abs/sqrt (with the NaN rule), set:distinct/difference/intersection/
has-same-node/leading/trailing (by node identity and document order), and
exsl:object-type. math:power and the transcendentals are omitted (they need a
C math library the Foundation-free target excludes); exsl:node-set awaits
result-tree-fragment support.
Part of #105 (Tier 3)
