US15 Simple procedures

[zjibben]

A procedure that does not reference outside variables is easier to offload to a device, or other off-chip processor.

PURE procedures operate without writing to global variables beyond their arguments, SIMPLE procedures would be further restricted from referencing global variables beyond their arguments.

• https://j3-fortran.org/doc/year/18/18-273r1.txt (initial proposal)
• https://j3-fortran.org/doc/year/19/19-201r1.txt (requirements, specifications, and syntax)
• https://j3-fortran.org/doc/year/20/20-116.txt (edits)

[klausler]

Another subtle benefit from using SIMPLE procedures is that SIMPLE procedure pointers don't need to waste space for a "static link" to potential host-associated objects; mere code address(es) suffice.