Support for returning the diff of import statements

[algomaster99]

Currently, the edit script doesn't return the diff between import statements from two versions of a compilation unit. This makes sense as import statements are not a part of the AST and the Diff object (the edit script) returns the AST changes only. However, it would be good to add this support as it will help in minimising the diff and hence, aiding to diffmin's purpose. See https://github.com/SpoonLabs/diffmin/issues/26.