Fix FromIterator for Segtree

[mousecrusher2]

This pull request fixes the FromIterator implementation for the Segtree struct. The previous implementation use Iterator::size_hint().0 to get length of the iterator. However, that is incorrect.

[TonalidadeHidrica]

You are correct. The current implementation, introduced by @mizar in #115, uses size_hint().0 as the number of elements in the segtree, but this is actually the lower bound of the element that is actually yielded from the iterator; moreover, it may be completely wrong. It's my fault overlooking it, thank you for pointing it out.

I guess however that my implementation in #107 is still correct. The implementation uses size_hint().0 to avoid reallocation when the number of elements is estimated correctly. It also fix the length of the slice for the parent nodes if there are not enough or too many of them compared to the number actually obtained by the iterator. Thus, I think this PR is superceded by #107, provided that I add the filter test that you suggested.

(By the way, despite #115 (comment), the FromIterator was not actually reverted. That's also my oversight...)

Comment me if someone (the OP or a maintainer) has any concern; otherwise I'm closing this PR.