Make Collector implementations shrink the result container if applicable

[seijikun]

Containers created by collectors are pre-allocated from the SizeHint. Though that's often not accurate and too large.
Shrink created container to actual size after collecting is done - if applicable?

[seijikun]

Decided against this. If the result is to be used afterwards, it would have to grow again.
If it is not used, one can manually shrink-to-fit.