Remove serde requirement from FRAME macros (#8628)

* 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 <gui.thiolliere@gmail.com>

substrate/bin/node/runtime/Cargo.toml

@@ -15,7 +15,6 @@ targets = ["x86_64-unknown-linux-gnu"]
 
 # third-party dependencies
 codec = { package = "parity-scale-codec", version = "2.0.0", default-features = false, features = ["derive"] }
-serde = { version = "1.0.102", optional = true }
 static_assertions = "1.1.0"
 hex-literal = { version = "0.3.1", optional = true }
 log = { version = "0.4.14", default-features = false }
@@ -133,7 +132,6 @@ std = [
 	"sp-core/std",
 	"pallet-randomness-collective-flip/std",
 	"sp-std/std",
-	"serde",
 	"pallet-session/std",
 	"sp-api/std",
 	"sp-runtime/std",