exposes Symbol.children

As requested in http://stackoverflow.com/questions/12078366/
adds ClassSymbol.baseClasses

Sure this stuff is also declared in Type.baseClasses, but this is
one of the hot paths in reflection (e.g. like ClassSymbol.typeParams),
so I think it's justified to have it as a dedicated method.

Unfortunately we cannot remove Type.baseClasses, because it includes
ridiculously involved calculation of base classes for compound types.
adds Symbol.isJava

Doesn't seem to be inferrable from the API we expose right now
Exposes Symbol.deSkolemize to macros
exposes MethodSymbol.isPrimaryConstructor

Doesn't seem to be inferrable from the API we expose right now
Exposes ToolBox.classLoader

By giving ToolBox.mirror the correct type and constructing the underlying
mirror from a proper classloader, we improve user experience w.r.t
runtime code generation.

Direct motivator: http://stackoverflow.com/questions/12122939, in which
instances of a toolbox-generated class are accessed with Scala reflection.
Fix SI-4581.

Specifically, the final flag on the generated static field
is no longer ommitted.

Fix 2 failing test-cases.
removes AbstractFileApi

no longer necessary, since scala.tools.nsc.io.AbstractFile
has been long moved to scala-reflect.jar
rehash of source file information for Symbol

Symbol.pos is moved to macros.Universe, because it's empty at runtime.

Also added Symbol.associatedFile, because it has valid usages during runtime.
When reflecting one might want to get to a classfile of the symbol and then
analyze it to e.g. decompile the bytecode or find out the associated src file.
Exposes Symbol.sourceModule

As discussed, this is not the best API to expose, because this is an
implementation detail that might change.

However the ship has sailed. We're already imposing
the moduleClass <-> sourceModule quirk to the users of our API:
Evidence: http://stackoverflow.com/questions/12128783.

There are reflection tasks that cannot be pulled without the knowledge
of this implementation detail, so we shouldn't pretend that we can
change it on a whim and not break anything.

Hence I propose to add sourceModule to the public contract
and bear the potential consequences.
definitive way to learn if a symbol is a val/var

I think `isVal` and `isVar` are the right names, because they
exactly map on Scala syntax.
SBT build now works with SBT 0.12.

Also fixed partest explosion that happened after partest redesign.
Fixes SI-6290 by creating real instnaces of language features.
Fix for SI-6034, covariant value classes.

My summary in the ticket was incorrect. The problem was that the
class type parameters were being cloned for the method and being
allowed to keep their variance. I threw in an assertion for anyone
attempting to create a method type with variant type parameters,
because hey, why should we allow such madness.
Merge pull request #1225 from jsuereth/fix/si-6290

Fixes SI-6290 by creating real instnaces of language features.
Merge pull request #1222 from jsuereth/fix/sbt-build

SBT build now works with SBT 0.12.
Merge pull request #1201 from axel22/issue/4581

Fix SI-4581.
Merge pull request #1227 from paulp/issue/6034

Fix for SI-6034, covariant value classes.
Suppressed 'possible cause' mis-warning.

I have seen this warning a bunch of times and it has not yet
been close to right.
Fix for SI-6263, futile adaptation.

Don't try to implicitly convert an unstable prefix
to a stable one by applying a view. As the matrix spoon kid
says, "that's impossible."