LES Part 1 (#3322)

* stub implementations of light client trait

* Light provider trait

* light client sync stubs

* LES boilerplate

* stub implementation of provider for client

* skeleton and request traits

* request definitions

* new_list -> begin_list

* handle unknown packet

* revise light implementation strategy

* make verification module public

* Move all light client work to own crate

* experiment with answering requests

* buffer flow scaffolding

* remove LESv2 requests

* buffer flow basics, implement cost table

* begin status module

* implement handshake parsing and creation

* implement announcement serialization

* errors, punishment, and handshake

* handle announcements

* making announcements, clean up warnings

* allow dead code temporarily