move to Required Associated Constants

[BabaBert]

This merge request transitions the trait methods of associated constants to actual required constants.
It also adds MANTISSA_DIGITS, DIGITS and RADIX to the Float traits.
This would close #213

[cuviper]

I'm not planning to process breaking changes anytime soon, so I'd rather leave this on the issue for now. A pull request will only bitrot over time. Thanks anyway!

[BabaBert]

I fully understand that.
However, is it an option to add these associated on top of the current methods? That would be possible and not break any existing functionality.

[tarcieri]

@BabaBert see #303 as an additive PR for traits with associated constants which was accepted

[BabaBert]

@tarcieri so I should rather create (a) new trait(s) that only contain the associates and implement that instead. That's a good point, but then the question arises how that trait should be called (FloatConst is already used) and how they should be split up. I would suggest having all the constants in one place, as they are related.