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 six worked examples of the C14N 1.0 Recommendation's section 3 run
as input/expected-output tests. They exposed and fixed: top-level
siblings now separated by single line feeds in the canonical form;
superfluous empty default-namespace declarations dropped in inclusive
mode (xmlns="" renders only when undeclaring a rendered non-empty
default); and scalar-level escape loops, so a CR clustered with a
following LF still escapes to 
. Escaping split to its own file
for the length caps.
Closes#132
