Introduces some structure for name lookups.

Too many unrelated things are intermingled.  If you
want to know what symbol corresponds to a particular name
in a particular context, you shouldn't have to involve
either a Tree or a Typer to find that out.  I toiled line
by line over typedIdent until it had shed its redundancies
and freed itself from the bowels of typed1.

The mechanism of name lookup is such that adding a
qualifier when the occasion calls for it is inseperable
without a lot more effort.  So to preserve a sane interface
I devised this small partitioning of outcomes.

  case class LookupSucceeded(qualifier, symbol)
  case class LookupAmbiguous(msg)
  case class LookupInaccessible(symbol, msg)
  case class LookupNotFound(msg)
Adds the core symbol lookup logic to Typers.

This unifies several disparate/ad-hoc mechanisms for
excluding symbols from eligibility in a single predicate.
This is the method on Context:

  def lookupSymbol(name: Name, qualifies: Symbol => Boolean)

The logic is largely that which was buried in typedIdent,
except that I fixed SI-3160 so that import foo._ does not
inject foo's private members into your namespace.
Made SilentResult more monadic.

Given that it's just a reimplementation of Option, we may as
well not also reimplement methods like map and getOrElse at
every call site.
Added some symbol lookup convenience methods.

Greasing the wheels for Typer's well-being.
Simplifying Typer.

Apply convenience methods to strip away complications.
Switch typedIdent to use Context's lookupSymbol.

This completes the transition.  Typer's bevy of special
cases to influence symbol lookup are encoded in its local
"qualifies" method, which it passes to lookupSymbol.
This allows access to be done correctly without infecting
Typer with such pedestrian concerns.
Adding some comments and clearer naming.
Tests for SI-3160, SI-4537, import precedence.
New take on SI-6534, value classes.

Don't prohibit equals and hashCode in universal traits;
instead, always override them in value classes.
SI-6488: Stop I/O threads prior to Process destruction
Fixes SI-5031 for separate compilation scenario.

When you have a conflicting member in package object and normal package that share the same namespace we remove the latter ClassSymbol from the scope. Now, this has an unpleasant consequence that companionClass/companionModule/companionSymbol no longer work correctly as they rely on finding the correspondent symbol using decls of the owner.
This fixes the problem of SI-5031 for separate compilation. Why the above change matters for finding foo.bar.Foo? Because when parsing the class we needed information about the static module (and we have the correct module symbol when completing the info). It's just that 043ce6d0565c9d5d960 relied on no longer valid assumptions. So we were getting NoSymbol and sym.exist was failing.

Obviously a more complete solution would be better if we didn't rely on the scope but that's too big to change for 2.10.0.
Make LookupNotFound a case object.

Turns out putting a group of case classes at what feels like
the top level might not be top-level enough, like if your "top"
is Analyzer and you wind up with different outer pointers in
every instance of Typer. Moved the whole bundle to SymbolTable.
SI-6581 fixed by inlining `Actor.self`.

This avoids the necessary type cast that was preventing leakage of internal migration classes.

Review by @phaller
Fixes SI-6559 - StringContext not using passed in escape function.

As reported by Curtis Stanford, with indication of what to fix.  standardInterpolator was not correctly
calling the passed in process function, so raw strings were not really raw.
Modification to SI-6534 patch.

Only exclude hashCode and equals from being overridden in
value classes, not other synthetics which may turn up such
as case class methods.
Fixes SI-6500 by making erasure more regular.

With the introduction of value classes, erasure uses specialErasure where a value class C with underlying type T is unboxed to an ErasedValueType. ErasedValue types are eliminated on phase later, in post-erasure. This was done everywhere, except in the parameter types of bridge methods. That was a mistale, because that way  bridge methods could not do the boxing/unboxing logic triggered by ErasedValueTypes.

Note: there is one remaining use of erasure (not specialErasure) in Erasure.scala. I put in a comment why that is OK.
Removing actors-migration from main repository so it can live on elsewhere.

* Removes actors-migration hooks from partest
* Removes actors-migration code
* removes actors-migration tests
* removes actors-migration distribution packaging.
SI-6556 no assert for surprising ctor result type

Previous fix to value classes uncovered some questionable cases in the backend where result types of constructor signatures are surprising. It's not a big deal because these types will be ignored afterwards anyway. But
the method uncovered some questionable situations which we should follow up on. However, breaking 2.9 code because of this is way too harsh. That's why the asserts were converted to warnings.

review by @paulp, @adriaanm
SI-6578 Harden against synthetics being added more than once.

Don't add synthetic methods if they already contain synthetic
methods from the set we're about to add.
Merge pull request #1537 from jsuereth/fix/SI-6559-2.10.x

Fixes SI-6559 - StringContext not using passed in escape function.