Make a way to spell literal local barriers

See merge request !130

Remove numpy dtypes from ArgExtFunction and SegmentedFunction

Closes #80

See merge request !124

Fix a 'threw StopIteration' warning in get_priority_tiers

See merge request !129

Preprocess the kernel before building the cache key

See merge request !127

Preschedule fixes

See merge request !128

preschedule may be interleaved with items not in the preschedule.

inside a subkernel. I'm not sure this heuristic is correct, because
it means that newly scheduled inames will always be scheduled inside
a subkernel.

target too.

so that we can avoid generating a dummy C function.

shapes which contain pymbolic expressions.

cacheable before creating the cache key.