Add pallet attribute macro to declare pallets (#6877)

* rename system Config to system Trait.

command used:
```
find frame/ bin/ test-utils/ utils/ -name *.rs -exec sed -i 's/system::Trait>::/system::Config>::/g' {} \;
find frame/ bin/ test-utils/ utils/ -name *.rs -exec sed -i 's/impl frame_system::Trait for /impl frame_system::Config for /g' {} \;
find frame/ bin/ test-utils/ utils/ -name *.rs -exec sed -i 's/impl system::Trait for /impl system::Config for /g' {} \;
```
plus some manual ones especially for frame-support tests and frame-system

* make construct_runtime handle Pallet and Module

pallets can now be implemented on struct named Pallet or Module, both
definition are valid.
This is because next macro will generate only Pallet placeholder.

* introduce pallet attribute macro

currently just with tests, frame_system and other example hasn't been
upgraded

* allow to print some upgrade helper from decl_storage

* Improved error msg, typo.

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Improved error msg, typo.

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Improved error message on unexpected attributes + ui test

* add test for transactional

* various typo

* some tips when spans are lost

* allow pallet to depend on other pallet instances

* make event type metadata consistent with call and constant

* error messages

* ignore doc example

* fix pallet upgrade template

* fixup

* fix doc

* fix indentation

* Apply suggestions code formatting

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* some renames + fix compilation

* remove unsupported genesis config type alias

* merge fixup

* fix ui tests

* additional doc

* implement StorageInstance with new syntax

* fix line width

* fix doc: because pallet doc goes below reexport doc

* Update frame/support/procedural/src/pallet/parse/event.rs

Co-authored-by: Andrew Jones <ascjones@gmail.com>

* Update frame/system/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update frame/support/test/tests/pallet_ui.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* improve doc as suggested

* revert construct_runtime Pallet part.

This revert the changes on construct_runtime. Now construct_runtime is
unchanged and instead pallet macro create a type alias
`type Module<..> = Pallet<..>` to be used by construct_runtime

* refactor with less intricated code

* fix ui test with new image

* fix ui tests

* add minor tests

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Andrew Jones <ascjones@gmail.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/frame/system/src/lib.rs

@@ -164,6 +164,7 @@ pub fn extrinsics_data_root<H: Hash>(xts: Vec<Vec<u8>>) -> H::Output {
 /// An object to track the currently used extrinsic weight in a block.
 pub type ConsumedWeight = PerDispatchClass<Weight>;
 
+/// System configuration trait. Implemented by runtime.
 pub trait Config: 'static + Eq + Clone {
 	/// The basic call filter to use in Origin. All origins are built with this filter as base,
 	/// except Root.
@@ -497,6 +498,11 @@ decl_error! {
 	}
 }
 
+/// Pallet struct placeholder on which is implemented the pallet logic.
+///
+/// It is currently an alias for `Module` as old macros still generate/use old name.
+pub type Pallet<T> = Module<T>;
+
 decl_module! {
 	pub struct Module<T: Config> for enum Call where origin: T::Origin, system=self {
 		type Error = Error<T>;
@@ -1343,3 +1349,14 @@ impl<T: Config> Lookup for ChainContext<T> {
 		<T::Lookup as StaticLookup>::lookup(s)
 	}
 }
+
+/// Prelude to be used alongside pallet macro, for ease of use.
+pub mod pallet_prelude {
+	pub use crate::{ensure_signed, ensure_none, ensure_root};
+
+	/// Type alias for the `Origin` associated type of system config.
+	pub type OriginFor<T> = <T as crate::Config>::Origin;
+
+	/// Type alias for the `BlockNumber` associated type of system config.
+	pub type BlockNumberFor<T> = <T as crate::Config>::BlockNumber;
+}