Add the string functions (concat, starts-with, contains, substring-before,
substring-after, substring, string-length, normalize-space, translate), the
number functions (sum, floor, ceiling, round), and the node functions
(local-name, name, namespace-uri, id, lang), merged into the evaluation
library. All pure Swift, no Foundation: substring search is hand-rolled.
Without a DTD, id() matches an attribute named id or xml:id.
Closes#41
