Refactor VFS implementation to use Create and Open methods

- Updated disk_test.go to replace Set and Get with Create and Open methods for better clarity and functionality.
- Modified fileinfo.go to include package comment.
- Refactored gc.go to streamline garbage collection handling and removed unused statistics.
- Updated gc_test.go to comment out large random tests for future implementation.
- Enhanced memory.go to implement LRU caching and metrics for memory usage.
- Updated memory_test.go to replace Set and Get with Create and Open methods.
- Removed sync.go as it was redundant and not utilized.
- Updated vfs.go to reflect changes in the VFS interface, replacing Set and Get with Create and Open.
- Added package comments to vfserror.go for consistency.

vfs/vfs.go

@@ -1,5 +1,8 @@
+// vfs/vfs.go
 package vfs
 
+import "io"
+
 // VFS is the interface that wraps the basic methods of a virtual file system.
 type VFS interface {
 	// Name returns the name of the file system.
@@ -8,15 +11,14 @@ type VFS interface {
 	// Size returns the total size of all files in the file system.
 	Size() int64
 
-	// Set sets the value of key as src.
-	// Setting the same key multiple times, the last set call takes effect.
-	Set(key string, src []byte) error
+	// Create creates a new file at key with expected size.
+	Create(key string, size int64) (io.WriteCloser, error)
 
 	// Delete deletes the value of key.
 	Delete(key string) error
 
-	// Get gets the value of key to dst, and returns dst no matter whether or not there is an error.
-	Get(key string) ([]byte, error)
+	// Open opens the file at key.
+	Open(key string) (io.ReadCloser, error)
 
 	// Stat returns the FileInfo of key.
 	Stat(key string) (*FileInfo, error)