LSPS2: Check OpeningFeeParams expiry times in no-std

[tnull]

In #53 we made the crate generally no-std compatible, with one small exception: we now don't have an immediate way to verify that OpeningFeeParams we receive as an LSP are still valid. Currently, we just don't check expiry in no-std builds, however, we should find a way to do this in the future, e.g., by letting the user giving us an implementation of a trait TimeSource or similar.