translate(string, from, to) ran from.firstIndex(of: character) for every
character of the source, so a large from/to map made it O(string * from). And
contains/substring-before/substring-after matched the needle at every position
of the haystack (Array.starts(with:) per index), O(n*m) and quadratic on a
repetitive string, which is a denial-of-service vector on untrusted input.
translate() now builds a from-character to replacement map (and a delete set)
once, first occurrence winning, and translates each source character with an
O(1) lookup. The substring search is Knuth-Morris-Pratt, O(n+m), which never
rescans. The results are unchanged.
A 4000-character large-alphabet translate drops ~0.13s to ~0.002s; a 16000-
character repetitive contains drops ~0.054s to ~0.0002s. The full suite, the
W3C XPath/XSLT and Apache Xalan corpora, and the WASM build stay green;
TranslateFunctionTests and SubstringSearchTests pin the semantics.
