(Issue #10) num_rational::Ratio: Add the ability to format as a decimal

[Hazardius]

Added simple formatting of Ratio as a decimal approximation to the fmt if it's given a Formatter with precision.

After is compiled and passed some new tests for formatting with precision in Formatter I've run the file through the cargo fmt before the commit.

As it's my first contribution in rust - I'm open for suggestions of improvements.

[cuviper]

Hi, thanks for the PR!

Some of your new where constraints are redundant -- Integer in particular will imply most of the rest. But sadly, adding constraints to this impl is a breaking change, so we can't do it exactly like this right now.

Maybe we can do it as a new method instead? Something like fn as_decimal(&self, precision: usize) -> String, although that implies std-only for String. Another option is to take a generic W: fmt::Write argument, but that's not as easy for the user. Yet another way is to return a formattable wrapper type, having that implement a decimal-style Display. Lots of possibilities...

I also worry that multiplying 10^n is going to be really prone to overflow, but we can think more deeply about that once the API is sorted out.

[Hazardius]

Thank you very much for your feedback.
I'll try to sit down and rethink it next week after Wednesday.

[Hazardius]

I was unable to sit with that issue before now, but here's silly separation of my code to the new as_decimal method.
Created a new trait for it and tried to remove as much of traits associated with T as I can.

[cuviper]

Why do you need Signed?

[Hazardius]

I'm pretty sure that I've added it to be able to use fn abs(&self).
Couldn't find a way to drop it now.

[cuviper]

This is implied by Display.
(via impl<T> ToString for T where T: Display)

[Hazardius]

Got rid of it from the type traits, but needed to add use std::string::ToString; elsewhere to make it compile.

[Hazardius]

Closing this PR as it's pretty much stagnant at this point.