IteratorRandom::sample: allow large "amount"

[stepancheg]

Summary

When requested amount is large but, but iterator is small, avoid allocating too large vec.

Motivation

If there's wrapper of such API (e.g. remote API), and it has parameter "sample size", because it is external API, user may pass very large value there, and if a user does that, it is better not to crash.

Details

This change is no-op where iterator lower bound is equal to iterator size.

• Added a CHANGELOG.md entry

[dhardy]

From a look at the Vec::extend code I think we don't need to reserve anything at all.

[stepancheg]

Yes, I think we should just avoid guessing capacity, because in many cases iterator should provide lower bound, and when it is not, we will do a few reallocations, not a big deal.

I updated the PR.

[dhardy]

Thanks for the PR