Container review

Added what was called a bap-container library.

Now it is split into several libraries:

- `Bap_dwarf` library to read dwarf information from file
- `Bap_elf` library to parse elf files (in a preliminary stage)
- `Bap_image` a memory representation of a binary file.

This is an internal view, externally the following modules are added:

- `Dwarf` implements dwarf parsing and function boundary identification (`Fbi`)
- `Elf` declares elf structure and parser, correspondingly `Elf.Parse` and `Elf.Types`
- `Image` to work with binary containers on a high level
- `Memory` and `Memory_exn` for accessing the memory of a loaded file image
- `Plugin` and `Plugins` to dynamically load `bap` plugins

P.S. I've tried to squash this commits as much as possible, but this is a minimum...