strings: Add benchmark test for trim function

strings: Add benchmark test for trim function

The strings.Trim function and variants allocate memory on the heap when creating a function to pass into TrimFunc.
Add a benchmark to document the behavior; an issue will be submitted to address this behavior in the compiler if possible.

Change-Id: I8b66721f077951f7e7b8cf3cf346fac27a9b68c0
Reviewed-on: https://go-review.googlesource.com/8200
Reviewed-by: Ian Lance Taylor <iant@golang.org>