Enhance rstsr-common API and add serialization for flags

[ajz34]

API breaking changes

• Refactored the flip methods for FlagSide and FlagUpLo enums to return the flipped value directly instead of a Result, since the operation cannot fail. Updated all call sites to remove unnecessary ? error propagation.

Enhancements

• Re-exported the BLAS traits prelude under the new feature in rstsr/src/prelude.rs for easier access.
• Made all BLAS flag enums (FlagOrder, TensorIterOrder, FlagTrans, FlagSide, FlagUpLo, FlagDiag, FlagSymm) derive Serialize and Deserialize, and added Serde rename attributes for better control over their string representations.

Crate structure

• Added serde as a dependency in the relevant Cargo.toml files and imported the necessary traits in flags.rs.
• Symlinked CHANGELOG.md to rstsr crate directory.

(Partially summarized by Copilot)