Offer parsing from bech32 strings

Offer parsing from bech32 strings

Add common bech32 parsing for BOLT 12 messages. The encoding is similar
to bech32 only without a checksum and with support for continuing
messages across multiple parts.

Messages implementing Bech32Encode are parsed into a TLV stream, which
is converted to the desired message content while performing semantic
checks. Checking after conversion allows for more elaborate checks of
data composed of multiple TLV records and for more meaningful error
messages.

The parsed bytes are also saved to allow creating messages with mirrored
data, even if TLV records are unknown.