secp256k1 Python library is not maintained anymore and coincurve was
already used in the `wire` module.
Changelog-None
Signed-off-by: Michal Rostecki <mrostecki@mailfence.com>
We are about to disect a couple of invoices for features, so let's add a class
that can encode and decode invoices from bolt11 strings. This is pretty much
the lnaddr.py file created by @rustyrussell with some minor changes. I'm
planning to clean this up further which is why I'm only exporting the
`Invoice` class for now.