Add i2u feature (#94)

[marcelbuesing]

Basically as discussed in the issue this adds a feature flag for encoding unsigned types in signed types.
Open question: does this require further changes in the decoder. Given the tests it seems to work without changes in the decoder.
I also could not figure out how to conditionally add a test for the case that the "u2i" is not set. Hints welcome.

e.g.

// this should not run when the u2i feature is set
#[test]
fn uint64() {
    let obj_min: EncoderResult<Bson> = to_bson(&u64::MIN);
    assert_matches!(obj_min, Err(EncoderError::UnsupportedUnsignedType));
}

[zonyitoo]

LGTM. cc @kyeah @saghm

[kyeah]

Looks great, thank you!

Open question: does this require further changes in the decoder. Given the tests it seems to work without changes in the decoder.

unsigned integers will be stored as signed ints in the database, so they will be decoded as signed ints using the usual deserialize_iXX methods.

I also could not figure out how to conditionally add a test for the case that the "u2i" is not set. Hints welcome.

You should be able to create default test cases, and ignore it if the feature is enabled:

#[test]
#[cfg_attr(feature = "u2i", ignore)]

should be good to merge after that 👍

[marcelbuesing]

You should be able to create default test cases, and ignore it if the feature is enabled:

Thanks! Just added the missing tests.

[zonyitoo]

@kyeah Merge it if you think it is Ok.

BTW. What if a crate uses bson with feature u2i, and the other crate in the same project doesn't?

[kyeah]

Thanks @marcelbuesing!

What if a crate uses bson with feature u2i, and the other crate in the same project doesn't?

Not sure! I'm a little rusty on how dependency resolution works in rust :)

[zonyitoo]

@marcelbuesing You can try this in your project, looking for feedbacks.