Providing size hint when creating a map allows avoiding re-allocating
underlying data structure if we know how many elements are going to
be inserted. This can be used for example during decoding maps in
gob.

Fixes #19599

Change-Id: I108035fec29391215d2261a73eaed1310b46bab1
Reviewed-on: https://go-review.googlesource.com/38335Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>