Remove duplicate imports

[chshersh]

Simpler version of #41 issue. Consider the following situation:

import App.Db (WithDb, query)
import App.Db (WithDb, execute)

It would be really good if smuggler can group modules with same names with join their import lists, like in the following snippet:

import App.Db (WithDb, execute, query)

Such situation often happen after resolving conflicts when rebasing over some branch. If such feature is implemented, then people will be able to just remove >>>>> git markers and smuggler will compress imports for them.