Add feature-gated support for casting unsigned integers to signed integers

[Ameobea]

When dealing with arbitrary inputs such as those from serde_json::Value, making use of the existing backwards compatibility APIs can be difficult or impossible. This PR introduces the ability to cast unsigned integers to i32 using the nightly try_from API. It exposes this behind an opt-in feature and doesn't make any backwards-incompatible changes.

If you'd rather avoid this conversion strategy and stick to strict type checking, that's fine; I'm making use of these for my own project and understand if you'll want to keep them out of upstream.

I also changed all instances of try!() to ?.

[zonyitoo]

LGTM. cc @kyeah

[kyeah]

Hi, sorry for the delay @Ameobea, @zonyitoo!

My intuition is that users should opt into these conversions for unsigned fields, in the same way that we have this compat module for de/serializing unsigned integers: https://github.com/zonyitoo/bson-rs/blob/master/src/compat/u2f.rs.

That is, we should add to_i32 and from_i32 conversions (along with _i64) instead of adding a global conversion flag — and if we do decide on a global flag, we should provide options for TryInto<i64> and TryInto<f64>.

What do y'all think?

[zonyitoo]

Yeah, you're right, that would be better.

[kyeah]

Hi @Ameobea! This question popped up again in #94, and it was noted that a global feature switch is useful to have when dealing with generated code.

#95 was opened and merged before I remembered that this was still open — sorry! I'm going to close this for now since the feature has been merged as u2i, but the try! conversions are still useful, and would be welcome in a separate PR.

Note that the u2i implementation is a little different — in particular I believe it converts u64 to i64 if possible.

Thank you for putting this PR together!