Refactoring the way sets work internally

Before this can be merged, all providers should be updated to use the
schema.Provider approach.

Next to that a migration path for state files should be added so the
state is converted to use the new internal representation of sets.

And maybe it would be nice to add some eye candy on top of this so
displaying a set on screen (in a plan for example) doesn’t show the
internally used hashes. But that is no requirement for merging this
code.
Temp fix for the AWS provider

This needs to be in here so TF will compile and all other providers can
be tested. This commit should be reverted before making the PR as the
AWS provider should be migrated to use the schema.Provider approach in
a separate PR.
Merge pull request #10 from svanharmelen/f-fix/change-set-logic

Fix/change set logic