Exists

Implements the function exists, which can be called on a given path
name.  If the given path does not exist, will return false.  If an error
was encountered, will inform the user of the error.  If both of these
cases are false, will return true (for the file exists).

This function is implemented in runtime code, in the file chpl-file-utils.c
and is called through a module interface found in FileSystem.chpl.
I added the test test/io/lydia/exists/fileOrDirExists.chpl, which tests
correct behavior when the file does not exist, when a file or directory
exists, and that a link to either passes.
Just a nit: you can do 'ln -s' to a file that doesn't exist, so there's not actually a need to create and remove going_away here.