Add `MaxEncodeLen` to `implement_per_thing!` (#10715)

* fix doc

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add MaxEncodedLen to implement_per_thing!

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Test that Percent et.al. can be used in Storage.

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add comment

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Revert "Test that Percent et.al. can be used in Storage."

This reverts commit 52558afea6f9b53fb07521f1b6393bc320d21dfc.

* Test MaxEncodedLen in implement_per_thing!

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Revert "Add comment"

This reverts commit 27a96f6e527f71a57ace289c041de514edbe9104.