Groom SetupPayload

[rwalker-apple]

Problem

SetupPayload's generation of the QRCode goes through 3 steps of
re-ordering of the information in the in-memory struct, making it harder
to verify correctness.

Summary of Changes

• change encoding of the struct fields from from big-endian to little endian
  in the bitfieldn
• remove the code that changes from a bitfield to a reversed array of unit16s
• remove the reversal of the encoded string chunks
• add a base45 decoder (for fun and for testing)

The resulting base45 string is little-endian, least significant bits in
the first character of the string.

[rwalker-apple]

@bhaskar-apple @sagar-apple

[bhaskar-apple]

LGTM. Had reviewed separately in Robs fork.

[sagar-apple]

add a base45 decoder (for fun and for testing)

Actually this is also part of the requirement for this library. To be able to decode a base45 string back into a SetupPayload. :)

[hawk248]

👍