Use procedural macros

[ascjones]

Closes #66.

Replaces the build_rpc_trait! macro with a trait level #[rpc] attribute. e.g.

#[rpc]
pub trait Rpc {
	/// Returns a protocol version
	#[rpc(name = "protocolVersion")]
	fn protocol_version(&self) -> Result<String>;

	/// Adds two numbers and returns a result
	#[rpc(name = "add", alias("callAsyncMetaAlias"))]
	fn add(&self, _: u64, _: u64) -> Result<u64>;

	/// Performs asynchronous operation
	#[rpc(name = "callAsync")]
	fn call(&self, _: u64) -> FutureResult<String, Error>;
}

and pubsub:

#[rpc]
pub trait Rpc {
	type Metadata;

	/// Hello subscription
	#[pubsub(subscription = "hello", subscribe, name = "hello_subscribe", alias("hello_sub"))]
	fn subscribe(&self, _: Self::Metadata, _: typed::Subscriber<String>, _: u64);

	/// Unsubscribe from hello subscription.
	#[pubsub(subscription = "hello", unsubscribe, name = "hello_unsubscribe")]
	fn unsubscribe(&self, _: Option<Self::Metadata>, _: SubscriptionId) -> Result<bool>;
}

Example of converting parity-ethereum.

Tasks

• Move public helper types currently in jsonrpc-macros, need to figure out where they should go
  • auto_args wrappers: can be done inline
  • delegates: move to core for now
  • pubsub: move to pubsub
  • Trailing replace with Option in last position
• Optional metadata in unsubscribe Optional metadata for unsubscribe. #352
• Copy tests from jsonrpc-macros and make them work
• Fix bug with single parameter as per: 00056d9#diff-61e49682a8be86b7288c62de1399069fR91
• Fix compilation error with single trailing arg
• Test with existing projects:
  • parity-ethereum
  • substrate
• Deprecate (rather than replace/remove) jsonrpc-macros with proc macros implementation currently in jsonrpc-derive. Need to find out how best to deprecate the macro or the entire crate.
• Rust 2018 compatibility (fn arg names and use crate?)
• Documentation!

Future Improvements

• Auto convert method names from snake case e.g. protocol_version -> protocolVersion, allow overrides as per serde.
• Improve error messages with proc-macro-diagnostics e.g. https://github.com/dtolnay/syn/blob/master/examples/lazy-static/lazy-static/src/lib.rs which is currently a nightly feature. Possible to bring it in already with ourt own `nightly' features with shims as per diesel
• Handle multiple trailing args, (already implemented here: https://github.com/paritytech/jsonrpc/compare/pzec_dependency_multiple_trailing_args)

[ascjones]

Now ready for full review @tomusdrw

[tomusdrw]

Looks good!