.length for staticlly typed arrays and fixup .length-type for Slices

simple addtion. 
works as expected.
That or add an assert to make sure we have the right type. Immagine a scenario with something like.
alias foo = int;
alias size_t = foo;

Which is a sensible may actually ends up happening as more targets are supporting. This kind of thing would fail in very bizarre way in the backend (as the backend assume that IntegerLiteral have an integer type). It used to have an invariant to check it, but I had to remove that as D do not have reentrant invariant.