git2 feature: Custom Odb backends

[tecc]

This PR is largely an attempt at making a "safe" API for adding custom ODB backends.
Additions made for this purpose have been concentrated in src/odb_backend.rs, with minor modifications to src/odb.rs and src/lib.rs to make it accessible.

A disclaimer, however: custom ODB backends lack documentation in libgit2, so most of the code and documentation (of which there is plenty) is based upon what I think the code does; if someone else knows better, feel free to tell me and I'll fix it.

I also haven't verified correctness nor safety, although I suspect the code does not violate all too many of Rust's memory rules.

The code has however been documented as thoroughly as I could be bothered to. It's not guaranteed to be correct, because libgit2 lacks documentation, but it should be largely correct as I cross-referenced with both libgit2 and libgit2sharp's source code and documentation to what extent I could.

Usage

This is largely untested (as of writing), so I may have made mistakes here and there.

1. Create a type that contains all the information you want from your backend (let's call it YourBackend).
2. Implement git2::odb_backend::OdbBackend trait for YourBackend. The only required method is fn supported_operations(&self) -> SupportedOperations - we'll come back to it.
3. Implement the methods you want your backend to support. All methods come with a great deal of implementation notes and recommendations.
4. In the supported_operations method, add a flag for each method your backend supports. The appropriate flags are described in each method's documentation.
5. To add YourBackend as a backend to an odb: Odb, use odb.add_custom_backend(your_backend, priority).
6. If you want to access YourBackend, you can do so with the CustomOdbBackend handle that was returned in the previous call.

TODO

There may also be some TODOs littered throughout the code.

• Implement the readstream and writestream methods
• Implement the foreach method
• Add more implementation notes; the documentation is currently very lacking.
• Create an example backend using this system. Perhaps try to reimplement one of libgit2's builtin backends?
• Combine duplicated bindings (most notably odb_backend::Indexer to indexer::Indexer and odb_backend::IndexerProgress to indexer::Progress)
• Reconsider the naming of some of the types and methods before merging