allows initializing WeightedShuffle with an ExactSizeIterator

[behzadnouri]

Problem

Using an ExactSizeIterator instead of a slice may allow to bypass the allocation to collect an iterator into a vector.

Summary of Changes

The commit allows initializing WeightedShuffle with an ExactSizeIterator.

[gregcusack]

looks good. but what do you mean by "may" here: "Using an ExactSizeIterator instead of a slice may allow to bypass the allocation to collect an iterator into a vector."

[behzadnouri]

looks good. but what do you mean by "may" here: "Using an ExactSizeIterator instead of a slice may allow to bypass the allocation to collect an iterator into a vector."

Not every iterator is an ExactSizeIterator.
So if you have an iterator which is not ExactSizeIterator you still need to collect::<Vec<_>>() before you can it pass it here.

[gregcusack]

lgtm!