SI-8280 regression in implicit selection.

In 2fa2db7840 I fixed a bug where applicable implicit conversions
would not be found for numeric types if one introduced any aliasing
or singleton types, for the usual reasons involving the absence of
uniform type normalization. See pos/t7228 for examples - that test
case has 20 errors in 2.10.3 but compiles in master.

An unintended side effect was making implicit search less oblivious.
It turns out that in so doing I had created ambiguity where there was
none before. Not because it was any more ambiguous, but because the
compiler now had the wits to notice the ambiguity at an earlier time.

The fix for this is not intuitive. The way the internal logic is,
we need to keep the wool over implicit search's eyes, which leads
to those unrecognized types being passed to adapt, where they are
recognized and weak subtyping suffices to be more specific. It is
sufficient for SI-7228 that weak subtyping be done correctly - the
other change, which is reverted here, was exposing the type arguments
of Function1 when a view exists as a subtype of Function1.

It is also possible this could be remedied by calling weak_<:<
somewhere which is presently <:<, but I don't know where and it
has a far greater chance of affecting something else than does
this, which is a straight reversion of a post-2.10.3 change.
Merge pull request #3524 from paulp/pr/8280

SI-8280 regression in implicit selection.