base-x generators

[chase-moskal]

we need to have generators at the low level so that hex/base58 etc can be streaming for gigantic conversions

maybe a new api is in order for this, maybe a breaking change,

hex.fromBytes(bytes) // returns an string iterator
hex.toBytes(string) // returns a byte iterator

hex(bytes) // returns a string
hex.toBuffer(string) // return Uint8Array

or for backwards compat,

hex.bytesIn(bytes) // returns an string iterator
hex.bytesOut(string) // returns a byte iterator

hex.fromBytes(bytes) // returns a string
hex.toBytes(string) // return Uint8Array