use pyutil as a separate package and contribute src/allmydata/util/* into pyutil

[zooko]

I'm posting this just as a way to get any feedback from Brian before I go and do this. My motivation is

(a) I'm tired of using cp/diff/path/vim to port improvements back and forth and merge changes among the current proprietary Mountain View source code, the Tahoe source code, and the separate ​pyutil project.

(b) I would like to make the contents of tahoe/src/allmydata/util/ available to a wider audience of Python programmers who might use them.

(P.S. I'm not entirely sure whether this is component "code" or "packaging". Oh well.)