Implement more formatting traits

[clarfonthey]

It'd be nice if every struct implemented LowerExp, UpperExp, Binary, Octal, LowerHex, and UpperHex.

These would make it a lot nicer to print things like BigInt and Ratio, which currently only have Debug and Display.

[clarfonthey]

See also: rust-lang/rust#39479.

[cuviper]

num-complex now has all of these.

num-bigint has all but LowerExp and UpperExp, which could actually be interesting for large numbers, especially if we can figure out how to do that quickly, without full digit-by-digit division.

num-rational still needs them all.

[cuviper]

Closing in favor of separate issues, rust-num/num-bigint#6 and rust-num/num-rational#1.