Change StringContext scaladoc

See e2a3498 commit message for explanation

Explicit private declaration better than implicit
Thanks to @som-snytt
scala.io.Source now implements java.io.Closeable

allows users to write a single function for dealing with Input/Output
streams and Sources. It also allows Source to be used in
"try-with-resources" statements in Java > 7.

No tests since this is a binary and source compatible change.
SI-8677 Duration: Zero - Inf should be MinusInf

Fixes #8677. Add basic tests.
Added example to Option.collect method.
Merge pull request #3857 from jxcoder/issue-3856

[scaladoc] Added example to Option.collect method.
Merge pull request #3853 from evvers/change-stringcontext-scaladoc

Change StringContext scaladoc
Merge pull request #3843 from Blaisorblade/topic/fix-minus-inf

SI-8677 Duration: Zero - Inf should be MinusInf
SI-8705   Added example to Option.contains method.
Merge pull request #3837 from hamnis/source-closeable

SI-8686: scala.io.Source now implements java.io.Closeable
[scaladoc] Changed align of example code to Option.contains and Option.collect methods.
Merge pull request #3864 from jxcoder/SI-8705

SI-8705   Added example to Option.contains method.
Bump version for 2.12.x

For this step, we still depend on _2.11 libraries (partest, xml,
parser-combinators).

After we release the first milestone of 2.12, we will update
versions.property to bootstrap to 2.12 dependencies.
WIP: trying to apply the Literal-based singleton types commit by @paulp to the master.
Fix non-working things.
Update check file.
Problems with null.
Update the check file.
This is needed for the tests to pass.
Added a macro to materialize an inhabitant for the singleton type.
Added a test with the Ranged class mentioned by @non.