[PersistentCollections] Introduce _Bucket, _Bitmap, _HashPath, and _H…

…ashValue

These provide nicer tools for slicing up hash values and dealing with node bitmaps.

_Bucket is a logical entry in the hash table inside a node.
_Bitmap is a set of bucket values, represented by a bitset.
_HashPath facilitates slicing up a hash value into a series of bucket values.
_HashValue represents a hash value.

This is still just mostly superficial refactoring, with little to no functional changes.

Sources/PersistentCollections/PersistentDictionary+Collection.swift

@@ -36,12 +36,12 @@ extension PersistentDictionary: Collection {
 
   /// Returns the index for the given key.
   public func index(forKey key: Key) -> Index? {
-    rootNode.index(key, _computeHash(key), 0, 0) 
+    rootNode.index(forKey: key, _HashPath(key), 0)
   }
 
   /// Accesses the key-value pair at the specified position.
   public subscript(position: Index) -> Element {
-    rootNode.get(position: position, 0, position._value)
+    rootNode.item(position: position._value)
   }
 }