Feature request: allow a user supplied function to deserialize to the repr.

[eparmstrong]

I'm dealing with files where the enums were sometimes serialized to floats instead of ints. I all ready have a function that I use with #[serde(deserialize_with = "path")] and would like to reuse it with Deserialize_repr. Maybe calling the attribute "deserialize_repr_with"?

The macro would then swap out
match <#repr as serde::Deserialize>::deserialize(deserializer)? {
with a call to the "deserialize_repr_with" function.