2.4.0 was a breaking change

[BatmanAoD]

The add_function signature changed, which breaks builds relying on this crate.

[MuZhou233]

The old signature is inconsist with document and blocks implemeting of key_get function. See #338
I don't have a way to not change the API. Do you have some idea?

[BatmanAoD]

@MuZhou233 I understand that the new API is necessary for certain features, but to avoid breaking people's builds unnecessarily, I think it would have been preferable to bump the version up to 3.0.

[hsluoyz]

@BatmanAoD this is a small feature and not many people are using it. So we will not trigger a major release for it.