Hinted uses

Hinted uses tie certain patterns - such as `List` or `[Test]` - to selected namespaces - such as `System.Collections.Generic`.

Hinted uses are endorsed on export:
- Howl source does not refer the hinted namespace.
- Resulting C# source refers the hinted namespace through a regular `using` statement.

Initially, non-configurable; later, configurable with an option to override language defaults and add custom rules. Configuration is specific to an assembly and/or a well defined physical (source tree) or logical (namespace) scope