New tests are added for creating, decoding, and re-encoding
litecoin invoices for both mainnet and testnet, as well as a test
that expects an error when the active network mismatches the
invoice.
This commit renames the invoice field Expiry to expiry, and changes
the type from time.Time to time.Duration. Getting the value of the
field will now have to be done using the getter Expiry(), which
will also return the default invoice expiry (3600s) if it is not set
explicitly by the the invoice.
