Implement BlocksClient for working with blocks (#671)

* rpc: Fill in any missing finalized blocks

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Move fill blocks test to RPC location

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* events: Remove the fill in strategy

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Introduce blocks client

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* client: Enable the block API

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Simplify `subscribe_finalized_headers` method

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Add tests for `subscribe_finalized_headers`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Implement `subscribe_headers`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Add tests for `subscribe_headers`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Move `missing_block_headers_will_be_filled_in` to blocks

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* events: Use the new subscribe to blocks

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Change API to return future similar to events

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* events: Use blocks API for subscribing to blocks

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Update subxt/src/blocks/blocks_client.rs

Co-authored-by: James Wilson <james@jsdw.me>

* blocks: Simplify docs for `subscribe_finalized_headers`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Use `PhantomDataSendSync` to avoid other bounds on `T: Config`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Add docs for best blocks

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Avoid one clone for the `client.rpc()`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Update testing/integration-tests/src/blocks/mod.rs

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* blocks: Improve `subscribe_headers` doc

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: James Wilson <james@jsdw.me>
Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

subxt/src/lib.rs

@@ -135,6 +135,7 @@
 
 pub use subxt_macro::subxt;
 
+pub mod blocks;
 pub mod client;
 pub mod config;
 pub mod constants;
@@ -148,7 +149,7 @@ pub mod tx;
 pub mod utils;
 
 // Expose a few of the most common types at root,
-// but leave most types behind their respoctive modules.
+// but leave most types behind their respective modules.
 pub use crate::{
     client::{
         OfflineClient,