Ensure contracts work properly on indexer properties

[JamesFaix]

Indexers should have parameter preconditions that apply to getters and setters, in addition to normal property preconditions on setters and postconditions on getters.

[JamesFaix]

Partial fix from PR #51. This makes contracts work as expected for indexers values, but preconditions on index values do not work. Tests are in place for index preconditions, but that may require deeper refactoring.