base36: Quickly convert to/from base36 values

For example: all numbers are stored at the noms level as base36-encoded strings, because noms' native integer support is atrocious and this compacts things fairly well.

If you want to get the noms' head's current height, cd into the noms dir, then:

$ noms show .::ndau.value.Height | base36
26267

The tool reads its value from its first argument if one is provided, or stdin otherwise. With the -E flag, it encodes base10 numbers to base36.