Add BIP32 key support; unify array newtyping; improve base58 trait

Sorry for so many things in one commit ... it was an iterative
process depending as I worked on BIP32 to get the other stuff
working. (And I was too lazy to separate it out after the fact.)

A breaking change by the array newtyping is that Show for Sha256dHash
now outputs the slice Show. You have to use `{:x}` to get the old hex
output.