SX strictly arity-checks lambdas; JS allows passing more args than
declared (extras accessible via arguments). Was raising "f expects
1 args, got 2" whenever Array.from passed (value, index) to a
1-arg mapFn. Fixed in js-build-param-list: every JS param list
now ends with &rest __extra_args__ unless an explicit rest is
present, so extras are silently absorbed.
conformance.sh: 148/148.
giles
bfec2a4320