Serialization of 2D rotations uses more data than strictly necessary

Currently, when serializing a 2-D rotation with serde, 2 (`Isometry2` with `UnitComplex` inside) or 4 (`Rotation2`) values are stored, but actually only one is needed (the angle). I think that, at least for `UnitComplex`, it is a waste to store the constant magnitude of 1.

I do not know how practical it would be to implement this special case in the current state of Rust without explicit specialization.

No files or tests are named. Start by locating the serde implementations for UnitComplex, Isometry2, and Rotation2, then assess whether an angle-only representation can be added without explicit specialization. Done would require a defined serialization format and coverage showing that the affected 2-D rotations serialize and deserialize correctly.

Written by the indexing model from the issue text.