Replacements should be able to add imports

[jackfirth]

If a replacement generates code that needs additional require statements to be added before it compiles, resyntax should be able to insert the needed imports. It should be possible to derive the necessary imports automatically by looking at the bindings of all of the identifiers in the syntax objects generated by a refactoring rule.

[jackfirth]

One tricky part here is how to display a suggested fix that adds imports. A fix on line 200 with an added import might want to insert the import on line 5. GitHub's suggestion comments in reviews would be an awful UI for that since it'd have to suggest modifying the entire span from line 5 to line 200.

One possible approach might be to gather up all added imports from all suggestions and make a single suggestion that adds all the imports. There are two downsides:

• Suggestions are no longer fully independent. Some suggestions would only work correctly if the import suggestion is also accepted.
• If some suggestions aren't accepted but the import adding suggestion is, unnecessary imports might be added.

Neither of these problems are that bad in my opinion. The first can be softened by including text in each suggestion warning the user when it requires adding imports. The second can be turned into less of a problem by building an analyzer that suggests removing unnecessary imports.