Issue 212 - refactor Checkable trait to be more generic (#287)

* runtime: refactor Checkable and BlindCheckable traits

* fix impl BlindCheckable for Extrinsic

* fix impl Checkable for TestXt

* fix impl Checkable for UncheckedExtrinsic

* fix tabs

* add ::Address to system::Trait since its no longer in Checkable trait

* replace tab by space in comment

* replace occurences of Checkable::check with ::check_with

* tx-pool: replace CheckedIntrinsic type alias since it now would require type param

* make more uses of Checkable compile

* adapt Executive impl to new Checkable trait

* fix that CheckedExtrinsic takes AccountId not Address as first type param

* Checkable trait: return error again since it's required in some cases

* Checkable: improve docstrings

* consistent punctuation and capitalization in docstrings

* Ctx -> Context

addresses https://github.com/paritytech/polkadot/pull/287#discussion_r200956240

* reduce trait bounds for impl Checkable for TestXt

addresses https://github.com/paritytech/polkadot/pull/287#discussion_r200839303

* use <UncheckedExtrinsic as Checkable>::Checked

addresses https://github.com/paritytech/polkadot/pull/287#discussion_r200955165

* Revert "add ::Address to system::Trait since its no longer in Checkable trait"

This reverts commit 02eb103015b833c995c9f9067aac2542bb7ce5ea.

* runtime/executive: properly fix that Address no longer in Checkable

* return `Result<Self::Checked, &'static str>` from `Checkable::check`