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/frame/example/Cargo.toml

@@ -13,7 +13,6 @@ readme = "README.md"
 targets = ["x86_64-unknown-linux-gnu"]
 
 [dependencies]
-serde = { version = "1.0.101", optional = true }
 codec = { package = "parity-scale-codec", version = "2.0.0", default-features = false }
 frame-support = { version = "3.0.0", default-features = false, path = "../support" }
 frame-system = { version = "3.0.0", default-features = false, path = "../system" }
@@ -36,7 +35,6 @@ std = [
 	"frame-system/std",
 	"log/std",
 	"pallet-balances/std",
-	"serde",
 	"sp-io/std",
 	"sp-runtime/std",
 	"sp-std/std"