Remove Cast from the IR altogether.

The last few remaining casts generated by the codegen were
converted into proper asInstanceOf's.

There is one remaining hack for RuntimeString, because
j.l.Strings are given to the $this param of RuntimeString
methods, which expect a sjsr.RuntimeString. To address this
without casts, I modified the IR subtyping test so that
j.l.String <: sjsr.RuntimeString. This is a hack, but it is
already less hacky than the cast. Hopefully we'll be able
to get entirely rid of this with proper static methods in the
future.