Allow multiple replacements from a single report?

As mentioned in #170, it can be useful to provide more than one suggested replacement in a single report, for things like adding imports associated with the primary auto-fix. Currently this requires multiple separate calls to `self.report()`, each with its own node and replacement, which would then be offered to the user as seemingly-unrelated changes and fixes.

Let’s consider allowing reports to offer multiple fixes that get applied as a single operation.