Add support for bstr::BString and git2::Oid, behind feature flags

[joshtriplett]

This adds support for SQL conversions for the bstr::BString binary
string type, and for the git2::Oid object ID type. Both types are
encoded and decoded as bytes.

These types are database-independent and work with any database that can
handle bytes, so add tests that work with sqlite.

[joshtriplett]

With this, Decode seems to work, but Encode doesn't compile when used with either BString or Oid. I think the bound on Encode needs to be something like &'q [u8]: ... rather than [u8]: ....

[mehcode]

Thanks! Got the impls merged and working. Only weirdness is I had to change the test because:

sqlite> SELECT 'abc123' is cast('abc123' as blob);
0
sqlite> SELECT  cast('abc123' as blob);
abc123
sqlite> SELECT cast('abc123' as blob) is cast('abc123' as blob);
1

[joshtriplett]

@mehcode Much appreciated, thank you! I look forward to using these impls once the next release comes out.