Allow the `ryu` dependency be opt-out

[kennytm]

ryu is Apache-2.0 which may be unacceptable for a MIT-only reverse dependency.

We could make ryu optional (and required by the arbitrary_precision feature), and make write_f32/write_f64 use the standard library's formatter when ryu is not used.

[dtolnay]

Thanks for the suggestion. The original C implementation my ryu crate is based on is dual licensed Apache 2.0 + Boost 1.0. Would it help if I make the ryu Rust code available under Boost 1.0? Boost is strictly less restrictive than MIT i.e. Boost is identical to MIT except that when distributing only "machine-executable object code generated by a source language processor" then there are no obligations whatsoever. I am not a lawyer but that seems like any MIT-only reverse dependency would be able to satisfy the terms of the Boost license.

[dtolnay]

Alternatively we could reach out to the original authors asking to make their code available to us under the terms of the MIT license.

[kennytm]

Law-wise I think following the original license choice (Apache-2.0/BSL-1.0) is a good idea (IANAL).

I raised this issue because it triggered rustc's license checker. So even with the license change, the tools would still need to know that BSL is compatible with MIT.

[dtolnay]

Done in ryu 0.2.6. Let's leave this issue open until the legal stuff is sorted on rustc's end.

[dtolnay]

I am not sure whether this is still moving or who is responsible for following up in the license checker tool, so I'll give this a close. Let me know if there is anything more needed on my end.