Add support for serialization of BitVec using serde

[mre]

This pull request adds support for serialization and deserialzation using serde.

It is based on the work of @HenningTimm in #50. I merely added the feature flag as requested by @pczarn in the comments of the previous pull request.

[pczarn]

We should forward the std feature to serde in two steps

• let's pass default-features = false to serde dep, so that the default is no_std
• forward the std feature to serde with

[features]
std = ["serde/std"]

[pczarn]

Thanks for the update. OK, I will merge as-is and add my own changes if testing shows they're necessary.

What's more, we might need CI testing of no_std.