adds Symbol.alternatives

As per Jason’s additional request, this commit exposes alternatives
in the base Symbol type, rather than only in TermSymbol.
adds initialize and fullyInitialize

As per Paul’s request, this commit introduces facilities to force initialization
of reflection artifacts.

Sure, initialize (actually, even fullyInitialize) is silently performed
by Symbol.typeSignature for runtime reflection, but people don’t have to
know about that.
adds Context.enclosingOwner

As per discussion at https://groups.google.com/forum/#!topic/scala-internals/nf_ooEBn6-k,
this commit introduces the new c.enclosingOwner API that is going to serve
two purposes: 1) provide a better controlled alternative to c.enclosingTree,
2) enable low-level tinkering with owner chains without having to cast
to compiler internals.

This solution is not ideal, because: 1) symbols are much more than
I would like to expose about enclosing lexical contexts (after the
aforementioned discussion I’m no longer completely sure whether exposing
nothing is the right thing to do, but exposing symbol completers is definitely
something that should be avoided), 2) we shouldn’t have to do that
low-level stuff in the first place.

However, let’s face the facts. This change represents both an improvement
over the state of the art wrt #1 and a long-awaited capability wrt #2.
I think this pretty much warrants its place in trunk in the spirit of
gradual, evolutionary development of reflection API.
adds internal.changeOwner

As per Jason’s and Mark’s request, this commit introduces `changeOwner`,
a facility to fixup symbol owner chains to prevent owner chain corruption
in macro expansions leading to crashes in LambdaLift and GenICode.

This is quite a low-level API that should only be used by expert users
to get their job done.

In the meanwhile we’ll be working on fixing the macro engine to
automatically prevent owner chain corruption in the first place:
https://groups.google.com/forum/#!topic/scala-internals/TtCTPlj_qcQ.