Make functionality to read relay state proof entries public

[4meta5]

Follow up for #1083 which makes the functionality to read arbitrary entries in the relay state proof public. We need this to read the BABE randomness values from the proof.

Let me know if there is a better way to do this @bkchr

[bkchr]

Why not directly expose the functions to read the babe randomness? As we do for all the other values?

[4meta5]

Then we will need functions for every value we would like to get. This will be at least:

read_current_block_randomness
read_one_epoch_ago_randomness
read_two_epochs_ago_randomness
read_epoch_index

It is less code to write one read_entry<T> which is generic over the successful result value returned.

Why not directly expose the functions to read the babe randomness? As we do for all the other values?

This approach also requires adding a new function if we ever want to read a new value from the RelayChainStateProof and that comes with a release delay so it is not preferred.

[bkchr]

I get what you say, but currently it also requires that you add these values before on the client side :P

We can add this function, I don't really care, but then please add some docs :)

[4meta5]

@bkchr Any suggestions to get the CI green? It succeeded on the only commit which changed the code, but is now failing after added doc comments.

error[E0425]: cannot find function `with_transaction_unchecked` in module `frame_support::storage`
     --> /usr/local/cargo/git/checkouts/polkadot-4038f27d5e4ea2e8/21f06c3/runtime/parachains/src/paras_inherent/mod.rs:606:31
      |
  606 |         ) = frame_support::storage::with_transaction_unchecked(|| {
      |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in `frame_support::storage`
  warning: ignoring -C extra-filename flag due to -o flag
  For more information about this error, try `rustc --explain E0425`.
  warning: `polkadot-runtime-parachains` (lib) generated 1 warning
  error: could not compile `polkadot-runtime-parachains` due to previous error; 1 warning emitted
  warning: build failed, waiting for other jobs to finish...
  error: build failed
warning: build failed, waiting for other jobs to finish...
The following warnings were emitted during compilation:
warning: `WASM_BUILD_WORKSPACE_HINT` env variable doesn't point to a directory that contains a `Cargo.lock`.

[bkchr]

@4meta5 can you please merge master