build: improve organization

[iboukhss]

@StokesAsselborn attempting to clean the build a little bit

Here is my idea:

src/
|--- cgi/         <- script execution: fork, execve
|--- config/      <- config parsing stuff: server_config.cpp/.hpp
|--- core/        <- core server: epoll, server.cpp/.hpp
|--- http/        <- http parsing, structs, etc.
|--- router/      <- routing stuff, connects with all other modules
build/
|--- obj/
|--- bin/webserv
tests/
|
...

I want to separate the modules more clearly so we have more room to work on different parts.