Make ElementsFeat an explicit member of Typed

With `typeDictSym _ = Nothing`, this disables constant folding and binding of ElementsFeat expressions.
Tweak Show instance for EPar.
Optimize a || skip and skip || b in Elements.
Optimize parFor 1 e in the same way as For 1 e.
Constant fold a few simple things.
Merge pull request #113 from pjonsson/elements

Elements improvements