Use derive_more crate to derive basic arithmetic traits

[JelteF]

I've created a library called derive_more that allows deriving of basic traits to decrease boilerplate code, such as From, Add, and Mul. With the recent release of Rust 1.15 it now works on stable rust.
I think this would be a crate that would benefit from using it. For instance this code could be derived with the library: https://github.com/rust-num/num/blob/master/complex/src/lib.rs#L442-L462

I don't mind making a pull request for this, but wanted to ask first i it is even wanted.

[cuviper]

First of all, we treat raising the rustc requirement as a breaking change. I know not everyone agrees with that policy, but I prefer the more conservative position. We might raise the bar a little for #257, or go ahead and break a lot for #261, not sure yet.

I think derive_more looks nice in general, especially for folks writing newtypes and enum wrappers. I'm just not sure we have enough use for it here to justify an extra dependency. I think Complex might be the only case that could really use it, and that boilerplate is pretty simple, and already written anyway.

[cuviper]

I don't think we'll do this without stronger justification.