Implement unexporting variables

[neunenak]

#1263

[casey]

Just took a look at this, mostly looks good. I think we should call this unexport, since unset sounds like it has something to do with settings.

There are a few tests we should have:

• Duplicate unexport statements should be an error. If people want duplicate unsets, for whatever reason, maybe because they're importing them from modules, we can relax this, but can't change things in the opposite direction, i.e., make duplicate an error when it hadn't been before.
• Having an export and unexport for the same variable should be an error. Similarly, this is easy to relax in the future, but we couldn't later make it an error if it wasn't one before.
• Recipes can export parameters with foo $VAR: syntax. I'm inclined to not make this an error, and we should have a test which checks that if you unexport a variable at the top level, but export it in a function parameter, it gets set.

[casey]

Doing an actual review so it shows up as requested changes in my PR lists.

[casey]

Dope, LGTM!

Did some very minor stuff. For example, you can do ${VAR:-unset}, to allow figuring out if a variable is unset without triggering an error, which is useful for tests.