Implement no_std support

[vks]

• Unfortunately, some functionality from core had to be vendored, because it is not exposed by stable Rust. Hopefully this functionality will be in the num_traits crate version 0.2. This would allow deleting the newly added traits.
• The code taken from Rust is MPL/Apache licensed. I think the BSD license is more permissive? I would suggest to relicense this crate under MPL/Apache, to reduce friction with the Rust ecosystem. Alternatively, the BasicFloat could be implemented in a separate crate with a separate license, or we just wait until its functionality is provided by num_traits.

[Eh2406]

Thank you so much! This is a really good improvement! Are you using my project for something!?

The code is BSD because it is derived from previous work that is BSD. If I re-license under MPL/Apache do I need to get permission from the persons I derived it from?

Does this compile on rust 1.0.0? If not what is the oldest version this works on? I rashly decided in my readme that changing minimum rust version is a breaking change. Which may be worth doing, but does require some care.

[vks]

Are you using my project for something!?

Not yet, but I might in the future. :) I want to have some way to do numerical integration on crates.io, so I'm interested in your crate. (I'll probably work on bindings or a port for cuba to have something for multidimensional integration.)

The code is BSD because it is derived from previous work that is BSD. If I re-license under MPL/Apache do I need to get permission from the persons I derived it from?

I'm not so sure myself. This blog post seems to suggest that it is possible to relicense BSD code as MPL/Apache (but not the other way around).

Does this compile on rust 1.0.0? If not what is the oldest version this works on? I rashly decided in my readme that changing minimum rust version is a breaking change. Which may be worth doing, but does require some care.

Unfortunately not, the minimum version required by num-traits is 1.8. I'll see whether I can get rid of the dependency.

[vks]

Removing the num_traits dependency gives me problems with f64::ln. It cannot be easily accessed with no_std on stable. :(

[Eh2406]

So I have re-licensed the project.
I'd love to see/help with a multivariable integration library. I'd prefer a port rather then bindings just for the ease of installation/bare_matal/wasm support. It should probably have good integration with ndarray.

Overall this looks go to me. The nits left:

• bump the min rust test in both CI
• bump the min rust in the readme
• update the change log to show the no_std in an unreleased breaking change
• set up some kind of reminder for when num_traits 0.2 is released

[vks]

See rust-num/num-traits#32 for the related PR.

[Eh2406]

Looks good so far, appveyor needs to be updated then I am good to merge.

[Eh2406]

Thank you again!