add try from methods

[benmkw]

as described in #85

I went ahead and implemented From and TryFrom for all applicable types, not just floats.

This is a more detailed approach than the current FromPrimitive Trait which always returns an Option but some conversions can never fail so we can unwrap them.

[vks]

Why is this dependency required?

[benmkw]

its for generating the call to the corresponding underlying impl ::[<from_ $name>]

                    fn try_from(n: $name) -> Result<Self, ()> {
                        <$into as FromPrimitive>::[<from_ $name>](n)
                            .map(Ratio::from_integer)
                            .ok_or(())
                    }

[vks]

As far as I can see, this does not include any new tests for the added functionality.

[benmkw]

As far as I can see, this does not include any new tests for the added functionality.

Yes, I was unsure how much this should be tested, maybe a few lines which check the Ok and Error case would be fine because it does not really add anything, just exposes existing functionality differently.

I was using this as a fork and also needed the try_into counterparts so I could add those as well if you think this would be a good direction.