((), Nil()) passes {
  case () => Cons(0, Nil())
}

was true(!) because Nil </: Cons and Cons </: Nil which caused the case
to filtered out silently.

Translation from String to List[Char] for Z3 on demand.

Added missing strings in enumeration.
Better error messages when tests fail.

corrected a web benchmark.

Removed warnings from SelfPrettyPrinter.

26d723e0 broke this, but I have no idea how it ever worked *after* that
change (it probably didn't). The problem is that untarring checks for
"malicious" tarfiles and in the process, normalizes entry paths. If the
outer path is not normalized, the subpath check fails.

Removed StringEscape
Removed z3 and other windows executables from the git index.

Fix semantics for substring
Simpler values for enumeration, externalised special values to disambiguation.
Removed the previous reference to leon.lang.string
Removed unused and redundant code.

Conflicts:
	src/main/scala/leon/xlang/ImperativeCodeElimination.scala