Function registry with support for external calls

See merge request !40

Improve loopy codegen determinism

See merge request !37

Use an ordered set for collecting operators to ensure deterministic
variable naming, as the variable name depends on the collection order.
(As an alternative I tried sorting the output of the collector by
string, but the stringification is very expensive on simple examples
like wave-min.)

Also, improve debuggability of any remaining bugs in loopy kernels
(such as any latent nondeterminism) by giving each assignment kernel a
separate name.

Avoid using specific Py3 sub-versions for CI

See merge request !34

Fix ToLoopyExpressionMapper for subscripted variables

See merge request inducer/grudge!32

Caches for speedy bind

See merge request !30

Placate flake8 3.6

See merge request inducer/grudge!28

Add Pytest/JUnit/Gitlab integration

See merge request inducer/grudge!27

Preinstall pybind11 for pyopencl

See merge request inducer/grudge!25

Use mpi4py.run in spawned MPI subprocessess

See merge request inducer/grudge!24

This reverts merge request !23