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.
The `test-junit` Eclipse project depends on `scala-compiler`

We want to write JUnit tests for Scala compiler so we should depend
on `scala-compiler` project in `test-junit`.

Also, make dependencies of `scala-compiler` project exported so it's
enough to depend on `scala-compiler` to get all the transitive
dependencies that are needed by the compiler. Otherwise, one would
need to copy all dependencies of Scala compiler to all Eclipse projects
that depend on the compiler.
Add example of SymbolTable unit test.

Add a SymbolTableTest which contains all the code needed to
initialize a SymbolTable in JUnit environment. It shows
that initialization of definitions works and one can easily
lookup some symbols and perform tests like subtyping tests.
Fix Platform type in Global to be JavaPlatform.

At the moment we have just one Platform: JavaPlatform in the
compiler. The whole Platform abstraction feels dubious and
Java backends need to downcast to JavaPlatform to implement some
optimizations. It seems like for now it's just better to fix platform
declared in Global to be JavaPlatform and get rid of downcasting.

I checked that even JavaScript backend declares itself as a subtype
of JavaPlatform so it seems like our abstraction is not that useful.

If we have an alternative platform with specific requirements we
might want to refactor our Platform abstraction again but for now
it seems dubious to pay a price for abstraction nobody uses.