Add helper library for handcompiling benchmarks and tests

[kyle-singer]

Adds a library (c2cilk) to the handcomp_tests such that it is easier to write "handcompiled" cilk code.
Certain features are written to make it a bit more annoying to use than otherwise could be achieved specifically because
we don't currently intend for this to be used outside of internal testing/benchmarking.

For internal benchmarking of opencilk, c2cilk makes it easier to write/port benchmarks to a handcompiled version such that we can test performance of cheetah in isolation of changes to the OpenCilk compiler. When compiled with -fopencilk, however, c2cilk will use the cilk_spawn, cilk_sync, etc., keywords, such that we can run roughly the same tests/benchmarks using the OpenCilk compiler.

I've tested the current handcomp_tests benchmarks on an x86_64 machine without avx, and there is no change in performance between the old handcompiled code and the handcompiled code using c2cilk.

[kyle-singer]

ABI changes require updates to this helper library (if we even want this still).