bip-0173: test vectors, HRP, and casing requirements updates

• a12uel5l: vector added to demonstrate that it and its uppercase version, A12UEL5L are both valid and treated as equal. segwit_addr.bech32_encode('a', [])

• 10a06t8: vector added to demonstrate that a combined empty HRP and empty data encode to a valid Bech32 string. segwit_addr.bech32_encode('', [])

• 1qzzfhee: vector added to demonstrate that an empty HRP encodes to a valid Bech32 string. segwit_addr.bech32_encode('', [0])

• pzry9x0s0muk: vector moved to a new section as it is misleading in the current section because "No separator character" can not affect checksum validity because the separator char is not used to compute the checksum.

>>> segwit_addr.bech32_create_checksum('', [1,2,3,4,5,6])
[15, 16, 15, 27, 28, 22]
>>> segwit_addr.bech32_verify_checksum('', [1,2,3,4,5,6]+[15, 16, 15, 27, 28, 22])
True

• 1pzry9x0s0muk: vector removed as incorrect because "Empty HRP" is Bech32 valid -- HRP validity is application specific. segwit_addr.bech32_encode('', [1,2,3,4,5,6])

• A1g7sgd8 and A1G7SGD8: vectors added to verify that implementations adhere to BIP requirement: "The lowercase form is used when determining a character's value for checksum purposes.". Note:

>>> segwit_addr.bech32_encode('a', [])
'a12uel5l'
>>> segwit_addr.bech32_encode('A', [])
'A1g7sgd8'  # BUG: violates "The lowercase form is used when determining a character's value for checksum purposes."

• 10a06t8: test vector added to demonstrate that while a valid Bech32 string, it is segwit application specific invalid.