Remove stale, commented out code from Incremental.scala

The commented out code was sitting here since 2010. It bit-rotted to the
point of not being useful anymore. Let's remove it.
Break Incremental.scala into smaller files.

Incremental.scala contained all three strategies of incremental
compilation:

  * ant-style
  * the default one (before name hashing)
  * name hashing

Let's move all those classes into separate files. Also, move common code
into a separate file.
Merge pull request #1360 from gkossakowski/incremental-cleanup

Incremental cleanup