When collecting into a ThinVec, pass the size hint to with_capacity

[joshtriplett]

This ensures that collecting from an iterator with a good size hint will avoid creating a ThinVec with excess capacity.

In particular, collecting 1-3 items into a ThinVec will no longer create a ThinVec with capacity 4, wasting proportionally significant memory. This can represent a large amount of overall memory usage in code that has a large number of small ThinVecs.

[nicoburns]

Seems reasonable to me. Although std Vec's implementation is quite a bit more sophisticated. The one thing we might consider doing here is only applying this is the iterator is non-empty. Also, apparently .extend is already applying this in most cases:

thin-vec/src/lib.rs

Line 2099 in 261cb50

impl<T> Extend<T> for ThinVec<T> {