add fun-op coroutine example

[adamstucci]

Simple functional big step semantics language demonstrating coroutines

[mn200]

• You need to put a reference to this new directory into the file
  src/parallel_builds/core/Holmakefile so that it is built as part of the regression machinery. Assuming it's pretty quick, it could go into the EXDIRS variable.

• Could you add a README to the directory explaining the basic approach? (Or a big comment at the head of the file would be OK too.)