Fix slow import of pyopencl

Multiple ORed values in CONSTANT_CLASSES.to_string

See merge request !100

Wait for Array.events in more places (elwise, reduction, scan, algorithms)

See merge request !102

Remove Py2.5 **dict(x=y) kwarg workaround

See merge request inducer/pyopencl!101

This is mostly implemented for `clGetKernelArgInfo` [1] called with
`CL_KERNEL_ARG_TYPE_QUALIFIER`. Since an argument can have multiple
type qualifiers (e.g. `const` and `volatile`), this returns the bitwise
or of the flags that we need to separate.

[1] https://www.khronos.org/registry/OpenCL/sdk/1.2/docs/man/xhtml/clGetKernelArgInfo.html

Add missing handling of cl.kernel_arg_info.TYPE_QUALIFIER

See merge request inducer/pyopencl!99

Mention oclgrind, intel-compute-runtime and beignet

See merge request !97

Use numpy 1.17.3 for python 3.8

Remove hashlib fallback for Py <2.5

See merge request !98

Rebuild from source if cached version causes BUILD_PROGRAM_FAILURE

Update docs for how to install system ICDs in Conda Forge

See merge request !96

Switch from array.vec to cltypes in image_from_array

See merge request !95

Implement get_async

Closes #18

See merge request inducer/pyopencl!91