Changes for substrate#3821

[expenses]

paritytech/substrate#3821 added a Debug derive to rstd, which we can use instead of #[cfg_attr(feature = "std", derive(Encode))]. It also broke some other stuff to do with Debug:

error[E0277]: `registrar::LimitParathreadCommits<T>` doesn't implement `core::fmt::Debug`
   --> /Users/ashley/projects/polkadot/runtime/src/registrar.rs:505:30
    |
505 | impl<T: Trait + Send + Sync> SignedExtension for LimitParathreadCommits<T> where
    |                              ^^^^^^^^^^^^^^^ `registrar::LimitParathreadCommits<T>` cannot be formatted using `{:?}`
    |
    = help: the trait `core::fmt::Debug` is not implemented for `registrar::LimitParathreadCommits<T>`
    = note: add `#[derive(Debug)]` or manually implement `core::fmt::Debug`

This should only get merged once polkadot-master gets updated and the branch changes from polkadot-master to master have been reverted.

[parity-cla-bot]

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Parity Technologies CLA Bot

[andresilva]

Sorry but I was already working on this and didn't notice this PR. The proper solution is to use a derive introduced in paritytech/substrate#3821 called RuntimeDebug. I opened a PR that updates to latest substrate-master branch (#491).