Make the SimplePass.Names unique for each function + rename prohibited names like "i" "x" "msg" "txt"

[dours]

1. Some very benign names (at least x, i, msg, txt) should not currently be used in EO code because of Argument name shadows return field name: is this a feature? objectionary/eo#416 , so a renaming is needed. Avoid these names in python tests before this issue is resolved.
2. SimplePass.Names are used incorrectly now, because an empty object is created before all passes in AST.Parse.parse. A new object with all statically known names should be created instead for each function.