Consider adding variants for Writeable write_len

[sffc]

Context: https://github.com/unicode-org/icu4x/pull/364/files#r511199181

We should consider making the write_len function of the Writeable trait return an enumeration expressing the confidence in the return value. For example:

pub enum WriteableLength {
    Exact(usize),
    Approximate(usize),
    LessThanOrEqual(usize),
    GreaterThanOrEqual(usize),
}

I would like to see more concrete use cases before introducing these additional variants.

[zbraniecki]

I think we'll need also "Unknown".

[sffc]

I think we'll need also "Unknown".

I struggle to think of a use case where that would be required; I think in most cases you can at least assert > or <.