Refactor the cake so SymbolTable does not depend on Global

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.

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