Consider including urbit keyfile generation

[Fang-]

A common use case for networking keys generated by this lib is creating a .key file for booting an urbit with. We might want to consider including logic for creating that file directly in this lib, instead of asking users to roll their own.

There's a case to be made that this is out of scope, but this comes up with some frequency, so just filing for the record at the very least.

[jtobin]

I was recently asked about this, so can probably take a look at it.