Implement @static annotation on singleton object fields.

This commit introduces the `@static` annotation on fields of static singleton objects.

A static singleton object is a top-level singleton object or an object nested within
a static singleton object.

The field annotated with `@static` is generated as a true static field in the companion
class of the object. The initialization of the field is done in the static initializer of the
companion class, instead of the object itself.

Here's an example. This:

    object Foo {
      @static val FIELD = 123
    }
    class Foo

generates :

   object Foo {
     def FIELD(): Int = Foo.FIELD
   }
   class Foo {
     <static> val FIELD = 123
   }

The accessor in `object Foo` is changed to return the static
field in `class Foo` (the companion class is generated if it is
not already present, and the same is done for getters if `FIELD`
is a `var`).

Furthermore, all the callsites to accessor `Foo.FIELD` are rewritten
to access the static field directly.

It is illegal to annotate a field in the singleton object as `@static` if there is
already a member of the same name in `class Foo`.

This allows better Java interoperability with frameworks which rely on static fields being
present in the class.
The `AtomicReferenceFieldUpdater`s are one such example. Instead of having
a Java base class holding the `volatile` mutable field `f` and a static field containing
a reference to the `AtomicReferenceFieldUpdater` that mutates `f` atomically,
and extending this Java base class from Scala, we can now simply do:

    object Foo {
      @static val arfu = AtomicReferenceUpdater.newUpdater(
        classOf[Foo], classOf[String], "f"
      )
    }
    class Foo {
      @volatile var f = null
      import Foo._
      def CAS(ov: String, nv: String) = arfu.compareAndSet(this, null, "")
    }

In summary, this commit introduces the following:
- adds the `@static` annotation
- for objects without a companion class and `@static` members, creates a companion class
(this is done in `CleanUp`)
- checks for conflicting names and if `@static` is used on static singleton object member
- creates the `static` field in the companion class for `@static` members
- moves the field initialization from the companion object to the static initializer of
the class (the initialization of `@static` members is bypassed in the `Constructors` phase,
and added to static ctors in `CleanUp`)
- all callsites to the accessors of `@static` are rewritten to access the static fields directly
(this is done in `GenICode`)
- getters and setters to `@static` fields access the static field directly, and the field in the
singleton object is not emitted (this is done in `GenICode`)
- changes in `GenJVM`, `GenASM` - now computing local var indices in static initializers
as well - this was an oversight before, now is necessary

Future work: allow the `@static` annotation on methods as well - this will
be added in a future commit.

Review by @odersky, @dragos, @paulp, @heathermiller.
WIP add private/lazy checks and a few tests.
Use `findMember` to lookup the static field in the host class.

The upcoming `findMember` optimizations should ensure that this
is fast enough.
Merge pull request #894 from axel22/topic/static-annot-cherry-2.10.x

Implement @static annotation on singleton object fields.