Parallelize MULTISHOT

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Right now, MULTISHOT experiments under the parallelization limit only use a single core. We could use this as an opportunity to do multicore computations. The general flow would be:

1. Is the computation under the parallelization limit? If no, perform single-threaded.
2. Does the computation exceed the number of shots necessary to parallelize? If no, perform single-threaded.
3. Parallelize across threads, each modifying thread-local state, then combine state.

This was requested by the Slack community.

[jlapeyre]

I can do this.
EDIT: I see you have "self-assigned". I'm happy to do it unless you want to do it yourself.

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@jlapeyre I've at least started it. I'll push my branch and maybe you can critique/discuss if it's in the right direction?

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By the way, I only tested this behavior on macOS.

[jlapeyre]

I'll try it this afternoon (was preparing and giving TTS today) on my linux laptop.

[jlapeyre]

I checked out this branch. grepping shows that I indeed have your edits. I then do make clean-cache and make clean (I needed them both in order for the build to succeed). Then make qvm. The resulting qvm binary is identical to the binary built from the master branch. This doesn't make sense. I'll look at it again tomorrow.
EDIT: I fixed this problem by uninstalling a quicklisp-installed version of qvm that was competing with the cloned repo.

[jlapeyre]

By the way, I only tested this behavior on macOS.

Can you give an example invoking qvm from the command line that will test this feature ?