Remove tools/make-release-notes

This has lived in scala/make-release-notes for some time.
Remove outdated build scripts.

Only used on the 2.9.x series:

    /code/jenkins-config grep -R epfl-build .
    ./jobs/scala-nightly-checkinit-2.9.x/config.xml:      <command>./tools/epfl-build 2.9.x -Xcheckinit</command>
    ./jobs/scala-nightly-main-2.9.x/config.xml:./tools/epfl-build 2.9.x &amp;&amp; ./tools/epfl-publish 2.9.x

    /code/jenkins-config grep -R epfl-publish .
    ./jobs/scala-nightly-main-2.9.x/config.xml:./tools/epfl-build 2.9.x &amp;&amp; ./tools/epfl-publish 2.9.x
Remove tools/updatescalacheck

We no longer maintain a fork.
Remove tools/test-renamer

It delivered us the terser 't1234' naming convention for test
cases. Its work is done.

This commit does the bidding of the script's author:

> Despite its shameful hackiness, checked in for posterity.
> It'll live on forever in the git history; then I can remove it.
Remove tools/remotetest

I suggest using the scala-checkin-manual Jenkins job instead.
Remove tools/*{cp, _scala, _scalac}, and tools/*pickled

The scripts to fire up a compiler or REPL from build products
have bitrotted just enough to get rid of them. They do not
include the correct JLine, nor the scala modules.

I've also removed showPickled and diffPickled which were
based on these, and were also based on some code in
ShowPickled that no longer exists since 48cc8b4.
SI-7938 - parallel collections should use default ExecutionContext

Parallel collections now use `scala.concurrent.ExecutionContext`
by default.

The `ExecutionContextTaskSupport` is optimized to use the
`ForkJoinPool` underlying the `ExecutionContext` if possible.
Otherwise, a fallback `TaskSupport` that creates a reduction tree and
execute an operation through `Future`s is used.
SI-7944 FOUND: stray undetermined type params in vicinity of implicits

Implicit search created a nested Context into which the results of
its typechecking, namely, errors and undetermined type parameters
(roughly: those inferred as Nothing) are stashed.

The code the drives the process was checking for errors, but
discarded those undetermined type parameters.

This commit copies them from the child context to the parent,
which lets `Typer#adapt` to get to:

    else if (hasUndetsInMonoMode) { // (9)
      assert(!context.inTypeConstructorAllowed, context) //@M
      instantiatePossiblyExpectingUnit(tree, mode, pt)
    }

Our lost TypeVar has found its way home! The reward for which
is being instantiated, based on another type inference session
adapting the expression's type to the expected type.
Collections: remove redundant calls to .seq

Students of Scala's history might recall that at introduction of
parallel collections, GenIterable et al were *not* added; instead
the parallel collections inherited from the existing interfaces.

This of course was an invitation to widespread disaster as any
existing code that foreach-ed over a collection might now experience
unwanted concurrency.

The first attempt to fix this was to add the `.seq` method to
convert a parallel colleciton to a sequential one. This was added
in e579152f732, and call sites in the standard library with
side-effecting foreach calls were changed to use that.

But this was (rightly) deemed inadequate, as we could hardly expect
people to change existing code or remember to do this in new code.

So later, in 3de96153e5b, GenIterable was sprouted, and parallel
collections were re-parented.

This commit removes residual needless calls to .seq when the static
type of the receiver is already a plain Iterable, which are no-ops.
Merge pull request #3089 from retronym/topic/opt-collections-seq

Collections: remove redundant calls to .seq
Merge pull request #3083 from retronym/topic/tools-autumn-clean

Cleanup tools/*
Merge pull request #3088 from retronym/ticket/7944

SI-7944 FOUND: stray undetermined type params in vicinity of implicits
from Issue #3098

from Issue #3098, some typos.
Merge pull request #3099 from yinxusen/patch-1

Fix typo in ParIterableLike
Merge pull request #3086 from axel22/topic/pc-execution-context

 - parallel collections should use default ExecutionContext