Add TranslationUnit.executor #14

Job	Run time
Documentation	4m 34s
Tests for downstream project Firedrake	22m 13s
Conda Examples	5m 4s
Tests for downstream project arraycontext	5m 39s
Tests for downstream project meshmode	24m 41s
Tests for downstream project grudge	16m 20s
Tests for downstream project pytential	46m 29s
Tests for downstream project pytato	10m 48s
Flake8	33s
Mypy	5m 28s
Pylint	6m 5s
Conda Pytest (ubuntu-latest)	10m 20s
Conda Pytest (macos-latest)	13m 44s
Conda Pytest with Intel CL	7m 19s
Conda Pytest without arg check	9m 1s
Conda Pytest Twice (for cache behavior)	9m 36s
	3h 17m 54s

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