Add missing reinterpret methods

[timholy]

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).

[timholy]

Superseded by #8, unless you don't like something about it.