Refactor the implementation of environments and implicit environments

[ppolesiuk]

Current implementation of implicit environment doesn't scale well into implicit declaration shadowing in presence of recursion. The following changes could be made.

• Merge (most of) functionality of Env and ImplicitEnv. It should simplify the management of implicit declaration and allow to get rid of name maps returned by Pattern.check_type (or at least these maps would not be necessary to implement shadowing; they still might be useful for implementing or-patterns).
• The environment could be responsible for detecting multiple definitions of the same name within e.g. block of recursive definitions. Current implementation based on syntactic checks (the Uniqueness module) is not able to detect multiple definitions of the same method. These check should be more semantic, because methods of the same name, but for different types should be considered different.