Added Summation&Product

[Gentil-N]

I have added two common operations : summation and product.

Even if we can use iterators, it seems that Python sum doesn't have any equivalent in Rust.

[At0micBee]

This is a good idea too, I've just been trying it out and for the most part it seems to work, I have however notice some improvements that could be done easily, to name a few:

1. If the stop value is smaller than the start value, the operations do not work
2. The operations do not work for floats; which isn't necessarily a problem, but ensuring it does work for them could be a good thing
3. Maybe move the macros to the range.rs module, which feels more appropriate for it

Thanks again!

[Gentil-N]

Yeah, so:

1. Instead of a couple "start&stop", I set a range expression: summation!(u32, n, 0..=10, n). Because, to process all possible types of range (where stop is smaller than start for instance) could be too heavy. With this method, the problem is redirected to users and we use the standard range.
2. I am not sure what you mean: if you talk about the formula into the operations, the following summation!(f64, n, 0..=10, n as f64 * 0.5) is working.
3. Ok, done!

You're welcome!

[At0micBee]

1. Great, that'll do!
2. Yeah you're right, I forgot I could do that...
3. Perfect.

Merging now.