Add `import ... hiding (...)` construct
Move checkRefIsValid up
Refactor ImportDeclaration

Introduce

  data ImportDeclarationType
    = Unqualified
    | Qualifying [DeclarationRef]
    | Hiding [DeclarationRef]
Add passing example for import mod hiding (..)
Fix error message for conflicting definition
Fix for psci after rebasing feature/import-hiding
Merge pull request #522 from andreypopp/feature/import-hiding

Add `import ... hiding (...)` construct