SI-7886 unsoundness in pattern matcher.

I tracked down what was behind the issue described here:

  // TODO: fix the illegal type bound in pos/t602 -- type inference
  // messes up before we get here:
  /*override def equals(x$1: Any): Boolean = ...
         // Span[Any] --> Any is not a legal type argument for Span!
         val o5: Option[com.mosol.sl.Span[Any]] =
  */

...which led straight to the unsoundness seen in neg/t7886.
It is dangerous to have an expected type of "Any" because
the type system will blithely ignore kind errors, since "Any"
can absorb anything. The consequence in this instance was
that inferring the case constructor for a type like

  Foo[T <: Bound]

if done with expected type Any, this would come up with Foo[Any].
I altered it to use expected type Foo[T], which lets the dummy
type parameter survive to carry the bound forward and restores
sense to the inference. The before/after output for -Xprint:patmat
on pos/t602.scala is:

15c15
<         if (x1.isInstanceOf[com.mosol.sl.Span[Any]])
---
>         if (x1.isInstanceOf[com.mosol.sl.Span[K]])
17c17
<             <synthetic> val x2: com.mosol.sl.Span[Any] = \
  (x1.asInstanceOf[com.mosol.sl.Span[Any]]: com.mosol.sl.Span[Any]);
---
>             <synthetic> val x2: com.mosol.sl.Span[K] = \
  (x1.asInstanceOf[com.mosol.sl.Span[K]]: com.mosol.sl.Span[K]);
19,20c19,20
<               val low$0: Option[Any] = x2.low;
<               val high$0: Option[Any] = x2.high;
---
>               val low$0: Option[K] = x2.low;
>               val high$0: Option[K] = x2.high;

A file in the library depended (needlessly) on the unsoundness.
It was easy to fix but reminds us this may affect existing code.
SI-6680 unsoundness in gadt typing.

Introduces -Xstrict-inference to deal with the significant
gap between soundness and what presently compiles. I'm hopeful
that it's TOO strict, because it finds e.g. 75 errors compiling
immutable/IntMap.scala, but it might be that bad.
Merge pull request #3005 from paulp/pr/7886

SI-7886 unsoundness in pattern matcher.