Add missing reinterpret methods

Since I suspect you don't do this much, a little cheat-sheet (apologies if this is all obvious to you): after merging, do
```
Pkg.checkout("FixedPointNumbers")
Pkg.update()
Pkg.tag("FixedPointNumbers")
Pkg.publish()
```
and that will bump the patch version for all users.

In my own packages I don't do this for every commit that comes in, but I'm not envisioning a whole stream of these FixedPointNumbers changes (but, who knows...still trying to finish migrating Images to use this).