Add LowerBounded/UpperBounded traits

[clarfonthey]

Potential solution for #208. With a breaking change, these could simply become supertraits of Bounded, but until then, we have to deal with blanket implementations.

I added both simply because it was easy to do so, although we could opt to not require both. I don't see it being that negative to include both, however.

[regexident]

Both trait LowerBounded and trait UpperBounded on their own look like a reasonable addition to num-traits. In light of the existing trait Bounded however they —and especially their blanket implementations— feel wrong to me as trait Bounded —like you said— should clearly be redefined in terms of trait Bounded: LowerBounded + UpperBounded {}, not the other way round.

The problem these traits aim to address is one of many arguments in favor of going thru with a future breaking release, while the proposed compromise/workaround might end up working against just that.

@cuviper either way this issue deserves an entry in the tracking issue, I think.

[cuviper]

I added that to the tracking issue. Otherwise, this looks fine to me, thanks!

bors r+

[bors]

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