Port a couple more RPC APIs to the new auto args (#2325)

* add auto-args deserialization for RPC

* make block param member public

* change BlockParam to a more generic Trailing<T> mechanism

* define work type

* build_rpc_trait macro, implement eth protocol

* fix up tests

* move eth_filter API to new macro

* port ethcore module to new rpc macro

* port ethcore_set to auto_args

* port net RPC to auto_args

* port rpc meta api to new

* skeleton for async RPC auto_args

* macro implementations for strongly-typed async RPC wrapper

* clarify docs

* reflect new required Rust version in README

[ci skip]