SI-6022 refactor to clean up model of variable equality modulo <:

this commit does a functionality-preserving refactoring
SI-6022 cleaner model of variable equality modulo <:

a more conservative "excludes": no need to reason about types

(TODO: check we don't get any spurious unreachability errors
 in the eclipse build, which is a good canary for this kind of thing)
Support per-group flags files in partest

In a directory-based test with file A_1.scala, scalac flags that should apply only to A_1.scala can be put in the file A_1.flags. Local flags are appended to global flags.
Rewrite of transformTypeTagEvidenceParams.

This fixes what a bug where the prefix seems to be checked but
is ignored; however I rewrote this method purely out of a desire
for tighter code.  I would consider it a great personal
favor if you would study the before/after and endeavor to
internalize the mechanisms I applied.

Points of particular note:

 - Before you write Option[Symbol], remember we already have
   an Option[Symbol], and it's Symbol.  None == NoSymbol.
 - flatMap can be to an Option, you don't have to create a
   one element List for flatMap's enjoyment
 - lastOption and headOption are rarely the way to go
   ... especially if you start with if (xs.isEmpty) return
 - The method which checks for emptiness is ".isEmpty",
   it is NOT ".length == 0"

I should add a pattern matcher warning for when you have a lower
case name in scope (like "contextParam") and then use a lower case
pattern variable of the same name, which of course ignores the
one outside the pattern and matches everything.

Review by @xeno-by.
SI-5940 impls are no longer in macro def pickles

The first officially released version of macros persisted
macro def -> impl bindings across compilation runs using a neat trick.

The right-hand side of macro definition (which contains a reference to an impl)
was typechecked and then put verbatim into an annotation on macro definition.

This solution is very simple, but unfortunately it's also lacking. If we use it
then signatures of macro defs become transitively dependent on scala-reflect.jar
(because they refer to macro impls, and macro impls refer to
scala.reflect.macros.Context defined in scala-reflect.jar).
More details can be found in https://issues.scala-lang.org/browse/SI-5940.

Therefore we have to avoid putting macro impls into binding pickles and
come up with our own serialization format. Situation is further complicated
by the fact that it's not enough to just pickle impl's class and method names,
because macro expansion needs knowledge about the shape of impl's signature
(which we can't pickle). Hence we precompute necessary stuff (e.g. the layout
of type parameters) when compiling macro defs.
macro implementations must be public

The immediate motivator for this change was the desire to keep things simple
during macro expansion.

Here's the story. When you expand a macro, you need to reflectively load
a macro implementation method and invoke it. However private methods in Scala
can sometime have their names mangled, so the reflector has to check multiple
variants of the same name (the normal one and the mangled one).

The problem is that since the previous commit we don't have an access to a
symbol of the macro impl, hence we cannot just use `impl.expandedName` like
we did before. Sure I could duplicate the logic of expandedName, but I have
a better suggestion.

Let's prohibit non-public macro implementations. This doesn't seem to hurt,
and it lets us avoid extra bit of complexity in Macros.scala.

If this measure turns out to be a hassle during the trial period of macros,
non-public macros can always be allowed. In fact, we can even have this feature
back for free when we migrate from Java reflection to Scala reflection for
invoking macro implementations (because Scala reflection knows how to account
for mangled private names). But that's the 2.10.x business.
Move side-effecting call out of debuglog(). Fixes SI-6223.

This change fixes a situation in which method calls aren't being properly
specialized. All the existing tests pass; in addition, it likely fixes other
specialization bugs (and improves performance of specialized code).

Also includes a small regression test.
Fix for view isEmpty.

Views have been inheriting the very inefficient isEmpty
from Traversable, and since isEmpty is not specifically
forwarded to the underlying collections, views miss out on all
the important optimizations in those collections which tend to
be implemented via method override.  Not to mention, they miss
out on correctness, because calling foreach has a habit of
forcing the first element of the view.
Merge pull request #1102 from phaller/partest-local-flags

Support per-group flags files in partest
Merge pull request #1100 from adriaanm/ticket-6022b

SI-6022 cleaner model of variable equality modulo <:
Merge pull request #1130 from non/si6223-2.10.x

Move side-effecting call out of debuglog(). Fixes SI-6223.
Merge pull request #1131 from paulp/topic/view-isempty

Topic/view isempty
accommodated the feedback for the pull request

For the discussion see: https://github.com/scala/scala/pull/1101
Merge pull request #1101 from scalamacros/ticket/5940

SI-5940 impls are no longer in macro def pickles