Optimization: avoid call to exists in PlainFile#iterator
Optimization: avoid call to exists in PlainFile#iterator

If we just established that the given path is a File or a Directory,
we can assume it continues to exist.

Before
----------------------------------
File.isFile calls           : 7620
File.isDirectory calls      : 2319
File.exists calls           : 5770

After
----------------------------------
File.isFile calls           : 7620
File.isDirectory calls      : 2319
File.exists calls           : 345

lrytz/scala,shimib/scala,slothspot/scala,martijnhoekstra/scala,jvican/scala,slothspot/scala,felixmulder/scala,felixmulder/scala,shimib/scala,lrytz/scala,scala/scala,jvican/scala,scala/scala,jvican/scala,scala/scala,martijnhoekstra/scala,lrytz/scala,felixmulder/scala,scala/scala,martijnhoekstra/scala,jvican/scala,lrytz/scala,shimib/scala,felixmulder/scala,shimib/scala,slothspot/scala,martijnhoekstra/scala,slothspot/scala,scala/scala,shimib/scala,scala/scala,felixmulder/scala,felixmulder/scala,jvican/scala,slothspot/scala,lrytz/scala,martijnhoekstra/scala,jvican/scala,shimib/scala,slothspot/scala,slothspot/scala,martijnhoekstra/scala,felixmulder/scala,jvican/scala,lrytz/scala