Lazily compute Iterator len

[Dr-Emann]

Lazily compute the length of an iterator, rather than doing so eagerly at creation time.

Additionally, add more efficient implementations of count, nth, and nth_back.

This required somewhat manually reimplementing std::iter::Flatten, but this is something we'll need to do anyway for #290 / #287.

This is currently built on top of #292, (so the PR currently includes commits from that PR too)

Fixes #294