SI-7897, SI-6675 improves name-based patmat

This emerges from a recent attempt to eliminate pattern matcher
related duplication and to bake the scalac-independent logic
out of it. I had in mind something a lot cleaner, but it was
a whole lot of work to get it here and I can take it no further.

Key file to admire is PatternExpander.scala, which should
provide a basis for some separation of concerns.

The bugs addressed are a CCE involving Tuple1 and an imprecise
warning regarding multiple pattern crushing.

Editorial: auto-tupling unapply results was a terrible idea which
should never have escaped from the crib. It is tantamount to
purposely throwing type safety down the toilet in the very place
where people need type safety the most. See SI-6111 and SI-6675 for
some other comments.
SI-7850 CCE in patmat with invalid isEmpty.

Name-based pattern matcher needed some hardening against
unapply methods with the right name but wrong types. Only
isEmpty methods which return Boolean are acceptable.
Catching it directly rather than indirectly also allowed
for better error messages.
SI-8045 type inference of extracted value

Test case for SI-8045, fixed by the preceding commits.
Eliminated some dead/redundant code based on review.
Finalized some case classes, for better static checking.
SI-8128 Fix regression in extractors returning existentials

The advent of the named based pattern matcher brought with it
a change in the way we determine the type of the value in the
"match monad". We used to take the base type to `Option` or `Seq`
(guided by the method name in `unapply` vs `unapplySeq`), and
simply use the type argument.

Name-based patmat, instead, uses the result type of methods in the
type. For example, the element type of an Option-like extractor
result is given by the result type of the no-args `get` method.

This approach, however, swiftly runs aground when navigating the
existential atolls. Here's why:

    scala> class F[_]
    defined class F

    scala> val tp = typeOf[Some[F[X]] forSome { type X }]
    warning: there were 1 feature warning(s); re-run with -feature for details
    tp: $r.intp.global.Type = scala.this.Some[F[X]] forSome { type X }

    scala> tp.baseType(typeOf[Option[_]].typeSymbol).typeArgs.head
    res10: $r.intp.global.Type = F[X] forSome { type X }

    scala> tp.memberType(tp.member(nme.get)).finalResultType
    res11: $r.intp.global.Type = F[X]

`res10` corresponds to 2.10.x approach in `matchMonadResult`.
`res11` corresponds to the new approach in `resultOfMatchingMethod`.

The last result is not wrapped by the existential type. This results
in errors like (shown under -Ydebug to turn un accurate printing of
skolems):

    error: error during expansion of this match (this is a scalac bug).
    The underlying error was: type mismatch;
     found   : _$1&0 where type _$1&0
     required: _$1
      (0: Any) match {
               ^
    one error found

This commit addresses the regression in 2.10.x compatible extractors
by using the 2.10 approach for them.

The residual problem is shown in the enclosed pending test.
Merge pull request #13 from retronym/ticket/8128

SI-8128 Fix regression in extractors returning existentials
Fix infinite recursion in name-based patmat.

I discovered on erroneous code it could fall into a loop looking
for product selectors, because ErrorType always claims to have
whatever member you're asking about.
Merge pull request #3275 from paulp/pr/patmat

Improves name-based patmat.