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/support/procedural/src/pallet/expand/mod.rs

@@ -0,0 +1,81 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+mod constants;
+mod pallet_struct;
+mod call;
+mod error;
+mod event;
+mod storage;
+mod hooks;
+mod store_trait;
+mod instances;
+mod genesis_build;
+mod genesis_config;
+mod type_value;
+
+use crate::pallet::Def;
+use quote::ToTokens;
+
+/// Merge where clause together, `where` token span is taken from the first not none one.
+pub fn merge_where_clauses(clauses: &[&Option<syn::WhereClause>]) -> Option<syn::WhereClause> {
+	let mut clauses = clauses.iter().filter_map(|f| f.as_ref());
+	let mut res = clauses.next()?.clone();
+	for other in clauses {
+		res.predicates.extend(other.predicates.iter().cloned())
+	}
+	Some(res)
+}
+
+/// Expand definition, in particular:
+/// * add some bounds and variants to type defined,
+/// * create some new types,
+/// * impl stuff on them.
+pub fn expand(mut def: Def) -> proc_macro2::TokenStream {
+	let constants = constants::expand_constants(&mut def);
+	let pallet_struct = pallet_struct::expand_pallet_struct(&mut def);
+	let call = call::expand_call(&mut def);
+	let error = error::expand_error(&mut def);
+	let event = event::expand_event(&mut def);
+	let storages = storage::expand_storages(&mut def);
+	let instances = instances::expand_instances(&mut def);
+	let store_trait = store_trait::expand_store_trait(&mut def);
+	let hooks = hooks::expand_hooks(&mut def);
+	let genesis_build = genesis_build::expand_genesis_build(&mut def);
+	let genesis_config = genesis_config::expand_genesis_config(&mut def);
+	let type_values = type_value::expand_type_values(&mut def);
+
+	let new_items = quote::quote!(
+		#constants
+		#pallet_struct
+		#call
+		#error
+		#event
+		#storages
+		#instances
+		#store_trait
+		#hooks
+		#genesis_build
+		#genesis_config
+		#type_values
+	);
+
+	def.item.content.as_mut().expect("This is checked by parsing").1
+		.push(syn::Item::Verbatim(new_items));
+
+	def.item.into_token_stream()
+}