* echo client/server example: dead code removal, add command line params, improve output messages
* Add nix-build result to .gitignore
* Merge echo-client and echo-server directories to echo-hs/ directory; rename main modules accordingly
* Use our protobuf compiler to generate bindings for the Echo service
* Simplify echo.proto to just one endpoint; use highlevel client/server interfaces and codegen for echo example
* Remove repetition counts and thread spawns from echo client, parameterize payload
* Update default.nix
