Implement more formatting traits

[cuviper]

LowerExp, UpperExp, Binary, Octal, LowerHex, and UpperHex are all reasonable, where the underlying type already implements them.

cc rust-num/num#259

[maxbla]

I'm not entirely sure what these formatting traits should do, as they seem to have been designed with only integers in mind. Would they format as decimals? fractions?

decimal	octal decimal	octal mixed fraction	octal improper fraction
1/8	0.1	1/10	1/10
8/3	2.525252...	2+2/3	10/3

The same questions apply to Binary, Octal, LowerHex and UpperHex. I think LowerExp and UpperExp are going to be a bit more tricky to implement, although they were designed with non-integers in mind, so that's good.

After writing the above table, I'm convinced we should use fractions, as otherwise we'll have infinite repeating decimals all over the place, and we would have to decide how to truncate them. I'm torn between mixed and improper fractions though. There is an argument to be made for mixed fractions, as in the case of a Ratio where the denominator is 1, it necessarily reduces to the integer behavior described in the trait (we could choose to not display denominators of 1 for both fraction methods). Both would be easy to implement, it's just about what users want/expect. Do they want 10000/3 or do they want 3333 + 1/3?

[cuviper]

We implement Display as decimal improper fractions, so I think other bases would be the same. Display also hides denominators of 1.

FWIW, the current implementation of Num::from_str_radix also only parses numer/denom. We probably should enhance that to parse plain integers too though (without a denominator).

[maxbla]

I would be happy to submit a PR for this issue, but maybe I'll wait until my other PRs get through. I want to make sure I see those through.