Verify Grandpa proofs from within runtime modules

[HCastano]

Moving this from paritytech/substrate#4167. The initial implementation from that PR was kind of messy, and should be redone with the correct Grandpa primitives.

[tomusdrw]

We will need grandpa proof verification as a builtin contract for Ethereum as well, so would be good to set nice API and crate boundaries right from the start.