zpay32: create new package for encoding/decoding payment requests

This commit adds a new package “zpay32”: which is used within the
daemon to encode/decode payment requests. A payment request currently
consists of: the public key of the payee, the payment hash to use, and
finally the amount to send over the network. The encoded payment
request consists of the mentioned fields concatenated to each other, a
cc32 checksum is added, then the blob is finally encoded using zbas32.
I call the resulting scheme “zpay32”.

A number of extensions may be explored in future commits including
adding a version byte, adding “hint” routing information,
cryptographically signed receipts and more,

glide.yaml

@@ -60,3 +60,4 @@ import:
 - package: github.com/btcsuite/btcd
   subpackages:
   - connmgr
+- package: github.com/tv42/zbase32