Remove Neg from BaseNum

[andersk]

As per rust-lang/rust#23945, Neg is no longer implemented for unsigned types.

[milibopp]

I think we should discuss in that case, what BaseNum is supposed to mean. See also #113.

cc @sebcrozet

[tomaka]

There are only two possibilities:

• Remove the implementation of BaseNum on unsigned types, which means that you won't be able to have a Mat4<u16> for example.
• Allow Mat4<u16>, but forbid all operations that may need to change the sign of the content of the matrix. That's what this PR does.

[milibopp]

I am perfectly aware of that. The question is whether u8, u16 etc. are supposed to be BaseNum. I tend to think that they ought to be as well, since that has been the case before.

But this still changes the abstract definition of BaseNum. Personally, I think that the algebraic definitions here should be more fine-grained anyhow. I just can't really judge, what exactly BaseNum is supposed to entail.

Anyway, to be practical I could probably merge this to get nalgebra to compile. Then we can think about proper abstractions later, see #32.

[tomaka]

Ahhh, yes. Could you publish it too?
The version on crates.io hasn't compiled for 3-4 days.

[milibopp]

I'm in the process of fixing everything right now.