Serde feature only works in combination with std

[jadamcrain]

https://github.com/billyrieger/bimap-rs/blob/3dca651620845a939ee9e5393c0a8fe9fe0a1656/src/lib.rs#L222C1-L223C15

I'd like to be able to use BiBtreeMap in combination w/ serde but without having std enabled. Should be possible to separate the btreemap and bihashmap serde impls, no?

Would you accept a PR separating the serde impls for the std vs no_std stuff?