Remove PARALLEL keyword as iteration order

[tehrengruber]

The PARALLEL keyword used as vertical iteration order in the computation definition is often misunderstood by users. As decided in the syntax workshop the PARALLEL keyword should be removed and computation() should be defined as:

def computation(iteration_order: Optional[IterationOrder] = None):
    pass

so it can be used with an empty computation():

with computation():  # Ok
    ...

During a transition period a deprecation warning should be printed.

[jdahm]

We discussed this in a team meeting and users seem to be put off by the proposal.

The feedback was that PARALLEL is like const in C++, it's useful because it's info the user is giving the compiler that it can use to determine if the user is making an error.

We now have a different proposal: keep the PARALLEL keyword as-is and also add an OIR optimization pass that converts FORWARD loops to PARALLEL if they can, and perhaps emit an info message saying the loop was converted.

This proposal is tracked in #1009.