Fix offchain election to respect the weight (#7215)

* Mockup

* Fix offchain election to respect the weight

* Fix builds a bit

* Update frame/staking/src/offchain_election.rs

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

* Update frame/staking/src/offchain_election.rs

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

* Make it build, binary search

* Fix a number of grumbles

* one more fix.

* remove unwrap.

* better alg.

* Better alg again.

* Final fixes

* Fix

* Rollback to normal

* Final touches.

* Better tests.

* Update frame/staking/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Proper maxExtWeight

* Final fix

* Final fix for the find_voter

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

substrate/frame/system/src/extensions/check_weight.rs

@@ -182,7 +182,7 @@ impl<T: Trait + Send + Sync> CheckWeight<T> where
 	/// Do the pre-dispatch checks. This can be applied to both signed and unsigned.
 	///
 	/// It checks and notes the new weight and length.
-	fn do_pre_dispatch(
+	pub fn do_pre_dispatch(
 		info: &DispatchInfoOf<T::Call>,
 		len: usize,
 	) -> Result<(), TransactionValidityError> {
@@ -198,7 +198,7 @@ impl<T: Trait + Send + Sync> CheckWeight<T> where
 	/// Do the validate checks. This can be applied to both signed and unsigned.
 	///
 	/// It only checks that the block weight and length limit will not exceed.
-	fn do_validate(
+	pub fn do_validate(
 		info: &DispatchInfoOf<T::Call>,
 		len: usize,
 	) -> TransactionValidity {