Fix Issue 7654 - Fix Rebindable of narrow strings.

`Rebindable!(const(T)[])` should be `T[]`, but `Rebindable` was using `ElementType` instead of `ElementEncodingType`, which would result in `dchar[]` for narrow strings. This pull fixes this simple mistake.

https://d.puremagic.com/issues/show_bug.cgi?id=7654