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.

exec_mapper_factory argument to bind()

This change adds the ExternalCall node and execution support for it.
External functions may be passed as part of the execution context.

Avoid using specific Py3 sub-versions for CI

See merge request !34

Fix ToLoopyExpressionMapper for subscripted variables

See merge request inducer/grudge!32