When the implicit scope is constructed an initial pruning phase attempts to
discard implicit members which it is believed would later be rejected as
ambiguous. Members of a companion are discarded if they can be reached via
more than one prefix, on the assumption that being visible via two or more
paths means that the multiple values must conflict with one another. However,
this does not take into account the possibility that the implicit members
might depend on the prefix in a way which renders them non-ambiguous. This
scenario is illustrated in scala/bug#4947 and
scala/bug#5340. This PR address that by only pruning
implicits which are independent of the prefix.

Whilst this PR tries to stay as close as possible to the current behaviour, I
believe that it would be desirable to drop the early pruning of even the
prefix-independent implicits. If we do that then we get improved error
messages reporting ambiguity rather the ambiguous members being silently
ignored which leads to otherwise unexplained failures of implicit resolution.
For instance in the case of `test/files/neg/t5340.scala` which currently
reports,

```
t5340.scala:15: error: type mismatch;
 found   : Quux[MyApp.r.E,MyApp.s.E]
 required: Int
  (new Quux[r.E, s.E]): Int // fails due to pre-stripping implicits which
   ^
one error found
```

we would get the following report,

```
t5340.scala:15: error: type mismatch;
 found   : Quux[MyApp.r.E,MyApp.s.E]
 required: Int
Note that implicit conversions are not applicable because they are ambiguous:
 both method conv in object E of type [T, U](b: Quux[T,U])Int
 and method conv in object E of type [T, U](b: Quux[T,U])Int
 are possible conversion functions from Quux[MyApp.r.E,MyApp.s.E] to Int
  (new Quux[r.E, s.E]): Int // fails due to pre-stripping implicits which
   ^
one error found
```

Which is clearly more informative (even if also a little baffling). I think
that ambiguity via multiple paths is sufficiently rare that the performance
benefits of this early pruning are likely to minimal and not enough to
outweigh improved error messages.

Because this change can result in a symbol _legitimately_ being revisited, via
a different prefix, we need to change the way that the a pruned path is
represented. Instead of using empty lists, as introduced in scala#5951 (which fixes
scala/bug#10081) we use explicit sentinels to mark
that a symbol has been visited at a given prefix yielding no additional
implicits. Testing this yielded scala/bug#10425
which appears to be a regression relative to 2.12.x. That's also fixed in this
PR.

When the implicit scope is constructed an initial pruning phase attempts to
discard implicit members which it is believed would later be rejected as
ambiguous. Members of a companion are discarded if they can be reached via
more than one prefix, on the assumption that being visible via two or more
paths means that the multiple values must conflict with one another. However,
this does not take into account the possibility that the implicit members
might depend on the prefix in a way which renders them non-ambiguous. This
scenario is illustrated in scala/bug#4947 and
scala/bug#5340. This PR address that by only pruning
implicits which are independent of the prefix.

Whilst this PR tries to stay as close as possible to the current behaviour, I
believe that it would be desirable to drop the early pruning of even the
prefix-independent implicits. If we do that then we get improved error
messages reporting ambiguity rather the ambiguous members being silently
ignored which leads to otherwise unexplained failures of implicit resolution.
For instance in the case of `test/files/neg/t5340.scala` which currently
reports,

```
t5340.scala:15: error: type mismatch;
 found   : Quux[MyApp.r.E,MyApp.s.E]
 required: Int
  (new Quux[r.E, s.E]): Int // fails due to pre-stripping implicits which
   ^
one error found
```

we would get the following report,

```
t5340.scala:15: error: type mismatch;
 found   : Quux[MyApp.r.E,MyApp.s.E]
 required: Int
Note that implicit conversions are not applicable because they are ambiguous:
 both method conv in object E of type [T, U](b: Quux[T,U])Int
 and method conv in object E of type [T, U](b: Quux[T,U])Int
 are possible conversion functions from Quux[MyApp.r.E,MyApp.s.E] to Int
  (new Quux[r.E, s.E]): Int // fails due to pre-stripping implicits which
   ^
one error found
```

Which is clearly more informative (even if also a little baffling). I think
that ambiguity via multiple paths is sufficiently rare that the performance
benefits of this early pruning are likely to minimal and not enough to
outweigh improved error messages.

Because this change can result in a symbol _legitimately_ being revisited, via
a different prefix, we need to change the way that the a pruned path is
represented. Instead of using empty lists, as introduced in scala#5951 (which fixes
scala/bug#10081) we use explicit sentinels to mark
that a symbol has been visited at a given prefix yielding no additional
implicits. Testing this yielded scala/bug#10425
which appears to be a regression relative to 2.12.x. That's also fixed in this
PR.