Allow hard-links to be created

[paeppa]

Would be great if cryptofs would also allow hard-links on disc. I made a "proof-of-concept" implementation (see patch attached) which only adds about 10 lines of code.

diff --git a/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemImpl.java b/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemImpl.java
index 3c84460..0fa5077 100644
--- a/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemImpl.java
+++ b/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemImpl.java
@@ -403,6 +403,12 @@
            throw new NoSuchFileException(cleartextSource.toString());
        }
    }
+   
+   void link(CryptoPath cleartextLink, CryptoPath cleartextExisting) throws IOException {
+       Path ciphertextLinkFile = cryptoPathMapper.getCiphertextFilePath(cleartextLink, CiphertextFileType.FILE);
+       Path ciphertextExistingFile = cryptoPathMapper.getCiphertextFilePath(cleartextExisting, CiphertextFileType.FILE);
+       Files.createLink(ciphertextLinkFile, ciphertextExistingFile);
+   }

    private void copyAttributes(Path src, Path dst) throws IOException {
        Set<Class<? extends FileAttributeView>> supportedAttributeViewTypes = fileStore.supportedFileAttributeViewTypes();
diff --git a/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemProvider.java b/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemProvider.java
index ea9595a..3627e68 100644
--- a/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemProvider.java
+++ b/src/main/java/org/cryptomator/cryptofs/CryptoFileSystemProvider.java
@@ -383,4 +383,9 @@
        }
    }

+   @Override
+   public void createLink(Path link, Path existing) throws IOException {
+       fileSystem(existing).link(CryptoPath.castAndAssertAbsolute(link), CryptoPath.castAndAssertAbsolute(existing));
+   }
+
 }