Performance of JrtFileSystemProvider.exists

[Jason Zaugg]

Recently I modified the Scala compiler to read class files
jrt:/ filesystem, rather than relying on rt.jar.

I noticed a slowdown when running on JDK 9 ea and exercising this code.
Profiles suggest that the bottleneck is in calls to Files.exists.
JrtFileSystemProvider inherits an implementation which needs to construct,
throw and catch NoSuchFileException to answer in the negative.

It is possible that I'm calling this more often than needed, and could
implement a caching layer. But it seems worthwhile for jrtfs to implement a
fast path for exists, as is done in UnixFileSystemProvider.

Possibly related bug "Startup regression with Jython":
https://bugs.openjdk.java.net/browse/JDK-8166236

Regards,

Jason Zaugg