Tweak implicit resolution

[odersky]

I have noted that the "more implicit parameters first" rule when applied
unconditionally can lead to implicit search explosion. The test case is
i3430.scala:

println(Nil.min) // error: ambiguous

We need to find an Ordering[T] for uninstantiated T. Before implicit arguments were taken
into account we'd try two types (in this case Long and BigInteger) and get an ambiguity. Then
we'd check whether any of the other candidates would dominate both of these types, which was
not the case. So, we are done with an ambiguity.

But when implicit arguments were taken into account there are many types that still are better
then these. E.g.

[T1: Ordering, T2: Ordering]: Ordering[(T1, T2)]

and so on far all other supported arities of Ordering for tuples! So one of these has to be tried.
That leads to searches for other uninstantiated orderings and so on. Running i3430.scala by hand,
I got a compiler hang. I am not sure why the test suite succeeded nevertheless; there must be
something surprising going on in the tests.

My fix to get round this issue is that now implicit parameters are only considered if the result
types of two alternatives are unifiable. Hopefully, that's not too constraining.

[odersky]

I solved the mystery why the neg/i3430.scala test was passing previously. We got:

-- Error: /Users/odersky/workspace/dotty/tests/neg/i3430.scala:3:9 -----------------------------------------------------
3 |  println(Nil.min) // error: no implicit found
  |         ^
  |         Recursion limit exceeded.
  |         Maybe there is an illegal cyclic reference?
  |         If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
  |         A recurring operation is (inner to outer):
  |         
  |           subtype Long <:< T
  |           subtype Ordering[Long] <:< Ordering[T]
  |           subtype scala.math.Ordering.Long.type <:< Ordering[T]
one error found

This PR contains a check file i3430.check that guards against relapsing in this way.

[odersky]

I have to merge this now as it blocks other PRs. We should continue to brainstorm about better solutions.