spec: generated TOC with linkable headers

This adds a table of contents to the spec, with a way to get the link of a
header on hover (like on GitHub) and a "return to top" small icon at the
end of each section.

Jekyll generates the H1-level table of contents, and then the current
one is expended using jQuery all the way down to H5 titles. Examples and
H6 headers are ignored.

GitHub's (MIT licensed) octicons are used for nice "link" and "up"
icons.
spec: a print stylesheet to remove the TOC when printing
spec: a header that links to the index, with a scala logo

...and a Scala favicon.
Bump timeout in interactive tests from 10s -> 30s.

Seeing too many of these failures on our build servers.

https://scala-webapps.epfl.ch/jenkins/view/2.N.x/job/scala-nightly-auxjvm-2.11.x/148/jdk=jdk8,label=auxjvm/console

This is most likely due to load on the machines.
Cleanup a few flags in test/files/

 * String interpolation isn't Xexperimental anymore

   A few useless Xexperimental flags in tests were left behind by 6917cca,
   after string interpolation was made non-experimental in 983f414.

 * things added under -Xfuture in 2.10 are very much Xpresent now, the
   flag isn't needed anymore.
Merge pull request #4007 from retronym/topic/bump-timeout

Bump timeout in interactive tests from 10s -> 30s.
Merge pull request #4009 from gourlaysama/wip/test-flags

Cleanup a few compiler flags in test/files/
Merge pull request #3996 from gourlaysama/wip/spec

spec: generated TOC, with linkable headers and return to top links
SI-8445, SI-6622 test cases, already fixed

They were most likely fixed in #3931 / e3107465c3.

The test case for SI-6622 is taken from Jason's PR #2654. I adjusted
the EnclosingMethod to be `null` in two places in the check file, for
the classes that are owned by fields (not methods).
SI-8868 Fix unpickling of local dummy symbols

These pop up as the owner of symbols in annotation arguments,
such as the ones introduced by the names/defaults desugaring.

The first two test cases here motivate the two patches to Unpicker.

The third requires both fixes, but exploits the problem directly,
without using `@deprecated` and named arguments.

See also 14fa7be / SI-8708 for a recently remedied kindred bug.
Merge pull request #4013 from retronym/ticket/8868

SI-8868 Fix unpickling of local dummy symbols
Merge pull request #4011 from lrytz/t8445-6622

SI-8445, SI-6622 test cases, already fixed
Don't remove elements from a map during iteration.

Doing so relies on implementation details which might change.

See #3911 / SI-8774.
Merge pull request #4018 from retronym/ticket/8774-concurrent-modification

Don't remove elements from a map during iteration.