SI-7492 Make scala.runtime.MethodCache private[scala]

These are internal implementation details which shouldn't be exposed
to users (and might go away when support for invokedynamic is added).
Improved testing framework for sets and maps.

Switched to JUnit testing framework for sets and maps.  They now test
broadly against each other for consistency.  Tests for mutable.AnyRefMap
and mutable.LongMap are folded in here (originals removed).  There is still
lots of redundancy with other tests that has not been removed.

This framework is also designed to enable more robust testing of changes to
implementations of sets and maps; although it's still quite possible to get
a broken implementation through, these tests should make it harder to get
the fundamentals wrong.
SI-7837 quickSort, along with Ordering[K], may result in stackoverflow because the code uses '==' instead of 'equiv'

== instead of equiv (from Ordering) was used by mistake.  Fixed.

Also created a test to make sure that == is not used by throwing an exception if it is (as suggested by Jason).
SI-8102 -0.0.abs must equal 0.0

SI-8102 points out that -0.0.abs returns -0.0 (in error). The same issue
exists for -0.0f. This commit fixes the issue for both by delegating to
math.abs.
SI-8120 Avoid tree sharing when typechecking patmat anon functions

When typechecking an empty selector `Match` corresponding to:

    { case ... => ... }: (A => B)

We wrap it in a `Function` and typecheck:

    (x$1 => x$1 match { case ... => ... })

Local symbols in this expression (representing values bound by
the pattern, or just definitions in the body or guard) are then
owned by the anonymous function's symbol.

However, if we ever discard this `Function` and start anew with
the empty selector match, as happens during the fallback to
use a view on the receiver in `tryTypedApply`, we found that we
had mutated the cases of the original tree, and allowed orphaned
local symbols to escape into the compiler pipeline.

This commit uses duplicated trees for the the cases in the synthetic
`Match` to avoid this problem.

`duplicateAndKeepPositions` is used to preserve range positions;
without this scala-refactoring PrettyPrinterTest fails.
`Tree#duplicate` uses offset positions in the copied tree, which
is appropriate when both the original and the copy are going to end up
in the final tree.
awakens default getter synthesis from the untyper nightmare

Our happy little macro paradise is regularly invaded by resetAllAttrs,
the bane of all macros and typers. It’s so ruthless and devastating that
we’ve been long scheming to hack something really cool and to one day
defeat it.

Today we make the first step towards the happy future. Today we overthrow
the UnTyper, resetAllAttrs’s elder brother that rules in the kingdoms of
GetterLand and CaseClassia, and banish him from the land of getters.
In the name of what’s good and meta, let’s band together and completely
drive him away in a subsequent pull request!

To put it in a nutshell, instead of using untyper on a DefDef to do
default getter synthesis, the commit duplicates the DefDef and does
resetLocalAttrs on it.

As default getter synthesis proceeds with figuring out type and value
parameters for the getter, then its tpt and finally its rhs, the commit
destructures the duplicated DefDef and then assembles the default getter
from the destructured parts instead of doing copyUntyped/copyUntypedInvariant
on the original DefDef.

I would say the test coverage is pretty good, as I had to figure out
3 or 4 test failures before I got to the stage when everything worked.
Iirc it tests pretty exotic stuff like polymorphic default parameters
in both second and third parameter lists, so it looks like we're pretty
good in this department.
untyper is no more

Unlike with default getters, removing untyper from case class synthesis
was trivial. Just resetLocalAttrs on a duplicate of the provided class def,
and that’s it.

resetAllAttrs, you’re next. We’ll get to you! Eventually...
Merge pull request #3305 from xeno-by/topic/copy-untyped

awakens default getter synthesis from the untyper nightmare
Merge pull request #3301 from Ichoran/issue/7837

Resolves SI-7837, failure in quickSort.
Merge pull request #3306 from Ichoran/topic/junit-set-map

Improved testing framework for sets and maps.
Merge pull request #3325 from clhodapp/fix/double.abs

Fix  (postfix abs for -0.0)
Merge pull request #3331 from retronym/ticket/8120

SI-8120 Avoid tree sharing when typechecking patmat anon functions
Merge pull request #3289 from soc/SI-7492-redux

SI-7492 Make scala.runtime.MethodCache private[scala]