Add Ant-style incremental compilation mode

This PR implements an Ant-style incremental compilation mode. This mode
emulates what Ant's scalac command does. It recompiles just changed source
files and does not perform any invalidation of dependencies.
This is a very naive mode of incremental compilation that very often leads
to broken binaries.

The Ant-style mode is being introduced because Scala team needs it for
migration of Scala compiler to sbt. The name hashing algorithm doesn't
work well with Scala compiler sources due to deep inheritance chains.
There's a plan to refactor compiler's code to use more composition instead
of inheritance.

Once Scala compiler sources are refactored to work well with name hashing
algorithm, Ant-style mode will be deleted immediately.