Need numeric trait for converting to `Option<uint>`

[lilyball]

In order to implement size_hint() on Range, we need some numeric trait that covers the ability to convert a given type to uint (or more generally, Option<uint>, to cover out of bounds conversions).

More specifically, we need to convert the result of a - b where a and b are values of the numeric type. The definition for this conversion to uint would be the number of times that you'd have to add One::one() to the result to get a back (which is the obvious definition, really).

This is needed because range() doesn't restrict its inputs to primitive numeric types.

[emberian]

Closing, ToPrimitive fills this gap.