Implement `Readable` for `Bolt12Invoice`

Implement `Readable` for `Bolt12Invoice`

In c13b6d814bfb5dd95f9e40f8ff07dd45c47a387e and
fc14495ac6a9dbe7dc9a6bd43c4395b599592db1 we implemented `Readable`
for `Refund` and `Offer`s, but didn't do so for `Bolt12Invoice`.

Because `Bolt12Invoice`s are not serializable to strings, we
generally expect users to read and write them as byte arrays, and
currently expose this via `TryFrom<Vec<u8>>`. This is fine, but
there's no reason to not implement `Readable` as well, and it
saves having to implement `TryFrom` in bindings.