Bring support for calling check_inherents (#490)

* Start

* More work

* Add proc-macro for `validate_block`

* Make everything compile

* Add some test

cumulus/pallets/parachain-system/proc-macro/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "cumulus-pallet-parachain-system-proc-macro"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+description = "Proc macros provided by the parachain-system pallet"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+syn = "1.0.73"
+proc-macro2 = "1.0.27"
+quote = "1.0.9"
+proc-macro-crate = "1.0.0"
+
+[features]
+default = [ "std" ]
+std = []