Change implementation of didSet observer of offset property in bit readers to a more efficient version (using _offset and ptr)

This requires making ByteReader._offset internal instead of private.

Author: tsolomko

Sources/ByteReader.swift

@@ -24,7 +24,7 @@ public class ByteReader {
         }
     }
 
-    private var _offset: Int
+    var _offset: Int
 
     var ptr: UnsafeBufferPointer<UInt8>
 

Sources/LsbBitReader.swift

@@ -257,7 +257,7 @@ public final class LsbBitReader: ByteReader, BitReader {
     public override var offset: Int {
         didSet {
             if !self.isFinished {
-                self.currentByte = self.data[self.offset]
+                self.currentByte = self.ptr[self._offset]
             }
         }
     }

Sources/MsbBitReader.swift

@@ -257,7 +257,7 @@ public final class MsbBitReader: ByteReader, BitReader {
     public override var offset: Int {
         didSet {
             if !self.isFinished {
-                self.currentByte = self.data[self.offset]
+                self.currentByte = self.ptr[self._offset]
             }
         }
     }