Add shift fuzzer

[disnet]

Shift recently released a really nice fuzzer. It would be great to add this as an npm script so we can start tracking down parser bugs.

[vendethiel]

Is it actually new? Isn't it @michaelficarra's fuzzer, updated / repurposed?

[disnet]

Yep the shift fuzzer is totally new, supports ES2015 (I don't think esfuzz has been updated for 2015), and was written by @bakkot + @michaelficarra.

[vendethiel]

Okay, thanks for the correction.

[gabejohnson]

@disnet I think this is going to require an upgrade to shift-*. It doesn't support shift-parser < 4.0

[gabejohnson]

To be clear, this is only an issue b/c version mismatch. If we could use a new version of shift-reducer just w/ the fuzzer then we would be fine.

[michaelficarra]

@gabejohnson With the way npm manages dependencies, is this actually an issue? You should be able to have many side-by-side incompatible installs of shift-reducer.

[gabejohnson]

@michaelficarra shift-codegen 4.0.0 is a top-level dependency. shift-fuzzer would requires version >= 5. The error I encountered re shift-parser was due to it being a dependency of shift-codegen (and node picking up v4). It's a moot point as @disnet is migrating to newer versions of shift-*.