Implement TrustedLen and FusedIterator for DistIter

[dhardy]

Fixes #618

[burdges]

ReadRng does not itself satisfy either TrustedLen or FusedIterator, right? It's okay because DistIter panics instead of propagating those errors though I guess.

[dhardy]

Panic is basically a type-safe way of avoiding the usual guarantees on return values — be it an "unexpected" failure of a supposedly infinite iterator, or a failure to allocate a new Box etc. So I see no issue here.