rpc: new frontend.

rpc: new frontend.

A new pass, connect_rpc, allows any HDL frontend that can read/write
JSON from/to stdin/stdout or an unix socket or a named pipe to
participate in elaboration as a first class citizen, such that any
other HDL supported by Yosys directly or indirectly can transparently
instantiate modules handled by this frontend.

Recognizing that many HDL frontends emit Verilog, it allows the RPC
frontend to direct Yosys to process the result of instantiation via
any built-in Yosys frontend. The resulting RTLIL is then hygienically
integrated into the overall design.