ppc: Rename to pps and refactor

[MikailBag]

old name: problem package compiling (not cool) new name: problem preparation system (or services, as you wish) (cooler)

new architecture: CLI and server

CLI starts server in background and sends RPC requests to the server. Server actually does stuff.

In future it will be possible to start one server which will process requests from many clients.

RPC protocol is based on HTTP POST requests and defined via Rust structs. In future, we should make consuming PPS api from other languages easier (e.g. provide JSON schema for types).

Also, it's worth extracting RPC implementation into jjs-commons. (UPD: done)

[MikailBag]

Also, in future pps will live in other repository in jjs-dev org.

[MikailBag]

bors r+

[bors[bot]]

Build succeeded:

• clippy
• codegen
• cpp
• hack
• py-fmt
• py-test
• style
• udeps
• unit-tests