Warn users when they use unsupported standard library imports

[rosieyohannan]

Can offer our targeted versions if relevant.

[pwaller]

This shouldn't be too hard to implement in some ways, but is a part of a broader story of how to pass errors out to users, and how we're going to make reco check work.

In terms of how the feature might look, at the moment importing any standard library package is likely to be problematic due to our general lack of support for many language constructs. At least, at the moment I'm not particularly aware of any package that will work.

Another way to approach the issue might be to fail only when unsupported language features are actually used. That is, packages may contain many features, some of which might work and others not. So we need to decide whether to disable standard library packages wholesale or not.