Added auto-tupling.

Auto-tupling should satisfy the following spec.

1. An application `f(args)` where `f` is a non-overloaded method which has a single, non-repeated parameter as its
first parameter list and where args consists of two or more arguments is expanded to `f((args))`.

2. A constructor pattern `C(args)` where `C.unapply` is a non-overloaded method which has a single, non-repeated parameter as its first parameter list and where args consists of two or more arguments is expanded to `C((args))`.

Auto-tupling can be disabled by language feature "noAutoTupling".

Conflicts:
	test/dotc/tests.scala
Removed explicit tuplings from dotc codebase.

Eliminated all "Dotty deviations" which were due to lack of auto-tupling.
Allow And/OrTypes in baseType operations

Introduce new cases for AndTypes and OrTypes in methods `derivesFrom`, `baseClasses`, and `baseTypeWithArgs`.
These cases are ultimately needed so that `baseTypeWithArgs` makes sense for union and intersection types.

Also, fixed embarrassing typo in method `TypeBounds#|`.
Merge refined types when distributing via "|".

Use the equality (where ~ is any form of refinement)

  T1 { x ~ R1 } & T2 { x ~ R2 }  ==  T1 & T2 { x ~ R1 & R2 }

We already did the same thing when distributing via "&".
Appromiximate union types by intersections.

Appromiximate union types by intersections of their common base classes. Controlled by option -Xkeep-unions.
If option is set, no approximation is done.

Motivations for approximating: There are two. First, union types are departure from Scala 2. From time to
time they lead to failure of inference. One example experiences in Dotty was in a foldLeft, where the accumulator
type was inferred to be Tree before and was now a union of two tree specific kinds. Tree was the correct type, whereas
the union type was too specific. These failures are not common (in the Dotty codebase there were 3, I believe), but
they cause considerable difficulty to diagnose. So it seems safer to have a compatibility mode with Scala 2.
The second motivation is that union types can become large and unwieldy. A function like TreeCopier has a result
type consisting of ~ 40 alternatives, where the alternative type would be just Tree.

Once we gain more experience with union types, we might consider flipping the option, and making union types the
default. But for now it is safer this way, I believe.
Removed annotations forced by union types

Removed annotations from the dotty codebase that were necessary because the inferred union types
caused inference errors later on.