Improved support for extension methods

[acizmarik]

This PR improves experience when working with extension methods. The main changes are:

• Directive @import should be used to define which namespaces should be searched for extension methods
• Namespace System.Linq was added to default imports
• Introducing ExtensionMethodsCache to cache found extension methods for individual namespaces

Method overload resolving was not changed

[exyi]

Very nice :) Please, just change the cached type and it's perfect.