AST support : Context sensitive auto-completions for exposing in import declarations

[rundis]

Summary:

When the user has positioned the cursor inside an exposing node of a import declaration an editor should be able to suggest only relative hint suggestions. Akin to the description in #239.

Discussion

• Need to have access to other modules AST so that one can list relevant nodes that are exposed from the module you are importing. This may be out of scope, but illustrates the need for gaining access to exposeds from a module. Both at top level and for ADTs

Details

• Need to be able to identify based on cursor position that you are in the exposed node of an import declaration node (and also if inside an exposed node of an exposed ADT!)
• Need to be able to list what is already exposed for an import (to be able to filter out dupes)
• Need to be able to list exposeds for any module and also for ADTs their exposed types