VerifySignature should be called in the BOLT11PaymentRequest ctor

[Sword-Smith]

As far as I can see, the signature is not verified by the constructor. Why not call VerifySignature iff ExplicitPayeePubKey != null and throw an exception if the signature does not match explicit pubkey?

Alternatively, the signature could also be verified in Parse/TryParse.

[NicolasDorier]

This is because of performance. Imagine you deserialize 1000th payment requests. Checking the signatures can be the difference between 100 ms and 10 seconds.