Implement `std::ops` for maths

[maciejhirsz]

Implement ops traits to make working with numbers easy:

let data = object!{ foo => 10 };

data["foo"] += 1;

assert_eq(data.dump(), r#"{"foo":11}"#);

[maciejhirsz]

~~I keep forgetting, this only works on the same types, so:~~

[maciejhirsz]

I've randomly looked into this again, and I was wrong (or right) about this. The math ops can be implemented for different types the same way PartialEq can be.