* Remove `serde` requirement from FRAME macros

Currently there is some implicit requirement on `serde` being present in
the `Cargo.toml` of a pallet when `GenesisConfig` is used. This pr
removes this requirement by using the serde attribute `serde(crate = "..")`.

* build a unique reexport of serde in impl_opaque_keys, by abusing paste doc concatenation

* Optimize

Co-authored-by: thiolliere <[email protected]>