Array event management: Support concurrent operations

[inducer]

• Add Array.concurrent_view() which creates a view with a copy of the .events list.
• Add Array.join_concurrent_view() to re-join those control flows.

[inducer]

Concurrent access to a single buffer is ill-formed in OpenCL 1, but allowed with fine- and coarse-grain SVM in OpenCL 2. So this is only relevant after #220.