Add a compiler option to write .sig files to disk

[retronym]

$ mkdir -p /tmp/out; rm -rf '/tmp/out/*'
$ qscalac -Youtline -Ypickle-java -Ypickle-write -Ystop-after:pickler -d /tmp/out sandbox/test.scala
$ find /tmp/out
/tmp/out
/tmp/out/p1
/tmp/out/p1/p2
/tmp/out/p1/p2/Test.sig
$ qscalac -cp /tmp/out -d /tmp/out sandbox/client.scala
$ find /tmp/out
/tmp/out
/tmp/out/p3
/tmp/out/p3/Client.class
/tmp/out/p1
/tmp/out/p1/p2
/tmp/out/p1/p2/Test.sig

$ tail sandbox/{test,client}.scala
==> sandbox/test.scala <==
package p1.p2

class Test {
  def foo = 42
}

==> sandbox/client.scala <==
package p3

class Client {
  new p1.p2.Test()
}

[retronym]

As discussed with @stuhood @lihaoyi.

Up to now, I've been managing writing of .sig files in my programatic compiler driver, https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/PipelineMain.scala

Hopefully this change will make it easier to experiment with this mode in Bazel/Pants.

TODO:

• Benchmark this mode with scala/compiler-benchmark for a few code-bases.
• Cleanup code reuse with the backend's classfile writer
• submit pr to scala/scala

[retronym]

Benchmarks show outline typing takes 12-20% of full compilation time. Explicitly declaring type annotations to members that form the API reduces the amount of work.

[retronym]

Merged to scala 2.12.x in scala#8175