Interpolate environment variables in imports

[rossabaker]

Fixes #53

[rossabaker]

A couple discussion points before we clear this for release:

• Changed types of public importsOf and resolveImports. I'm not sure that these really should be public in the first place. This is, as it stands, a breaking change.
• Should we be able to interpolate mutable names into imports, like config values and system properties? Or should import interpolation be limited to immutable values, like sys.env? I reused the binding interpolator here, but it might be considered too permissive when we start to think about things like file watching.

[timperrett]

@rossabaker im not wild on a major version breaking change for this functionality. Is there a way it can be done in a bin-compat manner? Perhaps we could (should?) just document the cases that dont work (not a huge fan of this, but i don't want to have to upgrade all the users right now)

[rossabaker]

This should pass MiMa (I'll add that later to be sure). It deprecates methods that neither worked nor are likely to be used.

Supports only environment variables in import directives. This is all the documentation ever promised (now made more explicit), and sidesteps all issues of how we monitor mutable bindings.

[timperrett]

@rossabaker im happy with this :-) thanks!