Refactor the cake so SymbolTable does not depend on Global

_The description of PR is copied from 364fcd9. All other commits are mostly supporting that big change._

This is rather large commit so I'll first explain the motivation behind it and then go through all changes in detail explaining the choices I made.

The motivation behind this refactoring was to make SymbolTable unit testable. I wanted a lightweight way of initializing SymbolTable and then writing unit tests for subtyping algorithm, various functionality related to Symbols, etc. All of that should be possible by precisely controlling what we test, e.g., create types and symbols by hand and not have them defined in source code as we normally do in partest (functional) tests.

The other motivation was to reduce and clarify dependencies we have in the compiler. Explicit dependencies lead to cleaner design. Also, explicit and reduces dependencies help incremental compilation which is a big problem for us in compiler's code base at the moment.

One of the challenges I faced during that refactoring was cyclic dependency between Platform and SymbolLoaders.

Platform depended on `SymbolLoaders.SymbolLoader` because it would define a root loader. SymbolLoaders depended on Platform for numerous reasons like deferring decision how to load a given symbol based on some Platform-specific hooks.

I decided to break that cycle by removing methods related to symbol loading from Platform interface. One could argue, that better fix would be to make SymbolLoaders to not depend on Platform (backend) concept but that would be much bigger refactoring. Also, we have a new concept for dealing with symbol loading: Mirrors.

For those reasons both `newClassLoader` and `rootLoader` were dropped from Platform interface.

Note that JavaPlatform still depends on Global so it can access phases defined in Global to implement `platformPhases` method.

Both GenICode and BCodeBodyBuilder have some Platform specific logic that requires casting because pattern matcher doesn't narrow types to give them a proper refinement. Check the changes for details.

Some logging utilities has been moved from Global to SymbolTable because they are accessed by SymbolTable. Since Global inherits from SymbolTable this should be a source compatible change.

The SymbolLoaders has dependency on `compileLate` method defined in Global. The purpose behind `compileLate` is not clear to me but the dependency looks a little bit dubious. At least we made that dependency explicit.

ScaladocGlobal and Global defined in interactive has been adapted in a way that makes them compile both with quick.comp and 2.11.0-M4 so my refactorings are not blocking the modularization effort.