switch to z-base-32

[zooko]

Switch from Python standard library base32 encoding to z-base-32 encoding.

The main advantage is that this eliminates trailing "=" signs. See the examples in

​https://zooko.com/repos/z-base-32/base32/DESIGN

Other advantages include that it uses an alphabet and a permutation thereof that make it slightly easier for humans to transcribe correctly, that it can nicely encode a number of bits that is not a multiple of 8, and that it uses lower-case by default. It also comes with an optimized C implementation, but we're not bothering to use that at this time.

[zooko]

fixed by 32c90c6468983f39

I would like to change it so that we use the z-base-32 package by package-re-use instead of by source-code-copying, but I'll get around to that later...