SI-6616 Check that unsafe operations are only called on the presentation compiler thread.

The method that checks the actual constraint is @elidable, expecting it to be used
for nightly builds but stripped-off in release builds. This way we don't lose any
performance, but 'fail-fast' in IDE nightlies.

This assumes that release builds will have at least `-Xelide-below ASSERTION`, but
this pull request does not do that.
SI-6539 Annotation for methods unfit for post-typer ASTs

Motivated by the `.value` method in the SBT task-syntax branch,
which should only be called within the context of the argument
to a setting initialization macro.

The facility is akin to a fatal deprecation.
Refine @compileTimeOnly

 - Don't default the message, and show it exclusively.
 - Fix cut-and-pasto in the @since tag
 - Be tolerant if the annotaion class is missing, as seems to
   have been the case compiling the continuations plugin.
 - s/\t/  / in the test file to show the errors are positioned
   correctly.
 - Use defensive getOrElse
show developer guidelines on opening pull request

Includes "Improvements and typo fixes to CONTRIBUTING.md" by @heathercmiller
as well as feedback from @retronym.
SI-6632 SI-6633 Fixes issues and data corruption in ListBuffer

 - Disallow negative positions for ListBuffer#insert/insertAll/update
 - Fix data corruption issue in ListBuffer#insert
sane printing of renamed imports

Having a select named "foo" with an underlying symbol named "bar"
and trying to make sense of all that by prettyprinting is very confusing
Merge pull request #1583 from dragos/issue/fix-6616

SI-6616 Check that unsafe operations are only called on the presentation...
Fix for SI-6662, macro failing too early.

Don't give up before you try tupling. Who knows what
someone might be doing with a Unit.
Merge pull request #1618 from scalamacros/ticket/6662

Fix for SI-6662, macro failing too early.
Merge pull request #1585 from retronym/ticket/6539-2

SI-6539 Annotation for methods unfit for post-typer ASTs
SI-5330, SI-6014 deal with existential self-type

This has been broken since https://github.com/scala/scala/commit/b7b81ca2#L0L567.
The existential rash is treated in a similar manner as in fc24db4c.

Conceptually, the fix would be `def selfTypeSkolemized =
widen.skolemizeExistential.narrow`, but simply widening before
narrowing achieves the same thing. Since we're in existential voodoo
territory, let's go for the minimal fix: replacing `this.narrow` by
`widen.narrow`.

--
Original patch by @retronym in #1074, refined by @paulp to
only perform widen.narrow incantation if there are
existentials present in the widened type, as
narrowing is expensive when the type is not a singleton.

The result is that compiling the entirety of quick, that
code path is hit only 143 times.  All the other calls hit
.narrow directly as before.  It looks like the definition
of negligible in the diff of -Ystatistics when compiling
src/library/scala/collection:

    < #symbols                      : 306315
    ---
    > #symbols                      : 306320
    12c13
    < #unique types                 : 293859
    ---
    > #unique types                 : 293865

I'm assuming based on the 2/1000ths of a percent increase
in symbol and type creation that wall clock is manageable,
but I didn't measure it.
Fixes SI-6150 - backport to 2.10.x branch.
Merge pull request #1612 from scalamacros/topic/showraw

sane printing of renamed imports
SI-6634 Fixes data corruption issue in ListBuffer#remove

This is the cut-down version with minimally invasive changes,
e. g. keeping the "auto-correcting" bounds algorithm.
Merge pull request #1593 from adriaanm/guidelines-2.10.x

show developer guidelines on opening pull request
Merge pull request #1624 from adriaanm/paulp-issue/5330-2.10.x

SI-5330, SI-6014 deal with existential self-type
SI-6551: don't insert apply call in polymorphic expression.

Don't rewrite an explicit apply method to dynamic polytypes.
SI-6663: don't ignore type parameter on selectDynamic invocation

Fix mkInvoke to handle selectDynamic calls of the form

  new C.foo[T].xyz

or

  new C.foo[T].xyz :U

(where C extends Dynamic)

Without this patch, the type parameter was silently ignored, and
possibly inferred to a different.

This patch fixes mkInvoke to handle these cases, where ctxTree has
the form Select(TypeApply(fun, targs), nme) or Typed(...)
Scaladoc update for collection.mutable.MultiMap

Addition of source code example on how to use a MultiMap and its
defined methods. Minor correction in documentation for method
`removeBinding`.
Restore the opimization apparently lost after merge.