Update uniform.rs

[avhz]

Add a standard() method to Uniform Distribution, following the same idea as the Normal::standard() method.

If you're happy with this change I can also add the same thing for other distributions that have an accepted 'standard' form.

[YeungOnion]

In #228 we also implemented the Default trait since that would contribute to API, i.e. adds functionality without newtype, so I'd like this to also impl Default if we have a standard or unit construction method and if you added those trivial tests, I'd merge that.

After all, if most users had to write a new_standard_normal()->Normal and new_standard_uniform()->Uniform I don't think that would break their development experience.

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So, wanting to ensure Default behaves expectedly (since most distributions don't have a T::new()->T), I am certain Normal and Uniform are unambiguous in their choice, I am less certain for others.

For example, Exponential distribution is parametrized by a single scale parameter, but I've not heard someone use the term, "standard exponential distribution"/"unit exponential distribution" even if I'd infer it to be $\lambda=1$, some arguments could be made for other distributions as well.

I suppose time and feedback will make this clear if other distributions have this, the author of #228 did not think so.

[avhz]

I've included Default and some basic unit tests as requested :)

[YeungOnion]

Looks good. Thanks for the contribution; hope you continue to provide feedback!