Move lru_cache definitions to __init__ (#20)

Using the lru_cache decorators on class methods, the ones that have a reference to `self`,
will also cache self. So we move it to the __init__ of the class

(DIS-2913)

dissect/fat/fat.py

@@ -160,7 +160,8 @@ def __init__(self, fh, fattype):
 
         self.entry_count = int(self.fh.size // (self.bits_per_entry / 8))
 
-    @lru_cache(4096)
+        self.get = lru_cache(4096)(self.get)
+
     def get(self, cluster):
         if cluster >= self.entry_count:
             raise ValueError(f"Cluster exceeds FAT entry count: {cluster} >= {self.entry_count}")