Container compiler

Compile the container to PHP code for best performances.

TODO:

- [x] ContainerCompiler
- [x] ValueDefinitionCompiler
- [x] AliasDefinitionCompiler
- [x] ClassDefinitionCompiler
- [x] CallbackDefinitionCompiler
- [x] Lazy reading/loading of definition sources (read definitions lazily bc the compiled container doesn't need them)
- [ ] Work on performances: [mnapoli/di-benchmark](https://github.com/mnapoli/di-benchmark)
- [ ] Compile all known definitions upstart in a single file

I did mess some things up with git, I should have put some commits in the 4.0 branch instead of this one :/