Add option to optimize the JVM for short-runnning applications

[smarter]

Ideally, dotc should reuse a resident compiler and we should not fork sbt
for every task. Until this happens, this option is useful for
development. Fixes #222.

Usage:
$ sbt -DOshort=""
$ ./bin/dotc -Oshort foo.scala

Review by @DarkDimius @odersky @namin .

[DarkDimius]

LGTM.
FYI: we are forking JVM just to use a different classpath. Dotty inherited classpath managing from scalac and the later requires to be able to find itself on the bootclasspath. I don't see reason for this to be required and I guess this is just an implementation detail. If you are interested on speeding up test suite even more you could try to find how to fix dotty so that it could run on normal classpath.

[retronym]

See retronym/dotty@7d91b5d for code that extracts the paths of the JARs that SBT has added to the classpath of the test run, using a secret handshake.

[retronym]

I notice a bug in that code, I forgot a call to getLines.

 ~/code/dotty sbt console
[info] Loading global plugins from /Users/jason/.sbt/0.13/plugins
[info] Loading project definition from /Users/jason/code/dotty/project
[info] Set current project to dotty (in build file:/Users/jason/code/dotty/)
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.11.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_25).
Type in expressions to have them evaluated.
Type :help for more information.

scala> Option(classOf[dotty.tools.dotc.config.PathResolver].getClassLoader.getResource("app.class.path")).map(cp => scala.io.Source.fromURL(cp).getLines.mkString(java.io.File.pathSeparator))
res1: Option[String] = Some(/Users/jason/code/dotty/target/scala-2.11/classes:/Users/jason/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.11.1.jar:/Users/jason/.ivy2/cache/org.scala-lang/scala-reflect/jars/scala-reflect-2.11.1.jar:/Users/jason/.ivy2/cache/org.scala-lang.modules/scala-xml_2.11/bundles/scala-xml_2.11-1.0.1.jar:/Users/jason/.ivy2/cache/org.scala-lang/scala-compiler/jars/scala-compiler-2.11.1.jar:/Users/jason/.ivy2/cache/org.scala-lang.modules/scala-xml_2.11/bundles/scala-xml_2.11-1.0.2.jar:/Users/jason/.ivy2/cache/org.scala-lang.modules/scala-parser-combinators_2.11/bundles/scala-parser-combinators_2.11-1.0.1.jar:/Users/jason/.ivy2/cache/jline/jline/jars/jline-2.11.jar)

scala> Option(classOf[dotty.tools.dotc.config.PathResolver].getClassLoader.getResource("boot.class.path")).map(cp => scala.io.Source.fromURL(cp).getLines.mkString(java.io.File.pathSeparator))
res2: Option[String] = Some(/Users/jason/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.11.1.jar:/Users/jason/.ivy2/cache/org.scala-lang/scala-compiler/jars/scala-compiler-2.11.1.jar:/Users/jason/.ivy2/cache/org.scala-lang/scala-reflect/jars/scala-reflect-2.11.1.jar:/Users/jason/.ivy2/cache/org.scala-lang.modules/scala-xml_2.11/bundles/scala-xml_2.11-1.0.2.jar:/Users/jason/.ivy2/cache/org.scala-lang.modules/scala-parser-combinators_2.11/bundles/scala-parser-combinators_2.11-1.0.1.jar:/Users/jason/.ivy2/cache/jline/jline/jars/jline-2.11.jar)

[smarter]

retronym/dotty@7d91b5d returns an error 404, I think you meant retronym@7d91b5d

[retronym]

Oops, that repo was private. I've just made it public.