Move iterator_range to a new iterator-utils.h file

Move iterator_range to a new iterator-utils.h file

A later patch will add more iterator-related utilities.  Rather than
putting them all directly in coretypes.h, it seemed better to add a
new header file, here called "iterator-utils.h".  This preliminary
patch moves the existing iterator_range class there too.

I used the same copyright date range as coretypes.h “just to be sure”.

gcc/
	* coretypes.h (iterator_range): Move to...
	* iterator-utils.h: ...this new file.