Introduce swap-or-not shuffle

See #563 for discussion.

specs/core/0_beacon-chain.md

@@ -697,53 +697,22 @@ def get_active_validator_indices(validators: List[Validator], epoch: EpochNumber
 
 ```python
 def shuffle(values: List[Any], seed: Bytes32) -> List[Any]:
-    """
-    Return the shuffled ``values`` with ``seed`` as entropy.
-    """
-    values_count = len(values)
+    indices = list(range(len(values)))
+    for round in range(90):
+        hashvalues = b''.join([
+            hash(seed + round.to_bytes(1, 'big') + i.to_bytes(4, 'big'))
+            for i in range((n + 255) // 256)
+        ])
+        pivot = int.from_bytes(hash(seed + round.to_bytes(1, 'big')), 'big') % n
         
-    # Entropy is consumed from the seed in 3-byte (24 bit) chunks.
-    rand_bytes = 3
-    # The highest possible result of the RNG.
-    rand_max = 2 ** (rand_bytes * 8) - 1
+        def permute(pos):
+            flip = (pivot - pos) % n
+            maxpos = max(pos, flip)
+            bit = (hashvalues[maxpos // 8] >> (maxpos % 8)) % 2
+            return flip if bit else pos
 
-    # The range of the RNG places an upper-bound on the size of the list that
-    # may be shuffled. It is a logic error to supply an oversized list.
-    assert values_count < rand_max
-
-    output = [x for x in values]
-    source = seed
-    index = 0
-    while index < values_count - 1:
-        # Re-hash the `source` to obtain a new pattern of bytes.
-        source = hash(source)
-        # Iterate through the `source` bytes in 3-byte chunks.
-        for position in range(0, 32 - (32 % rand_bytes), rand_bytes):
-            # Determine the number of indices remaining in `values` and exit
-            # once the last index is reached.
-            remaining = values_count - index
-            if remaining == 1:
-                break
-
-            # Read 3-bytes of `source` as a 24-bit big-endian integer.
-            sample_from_source = int.from_bytes(source[position:position + rand_bytes], 'big')
-
-            # Sample values greater than or equal to `sample_max` will cause
-            # modulo bias when mapped into the `remaining` range.
-            sample_max = rand_max - rand_max % remaining
-
-            # Perform a swap if the consumed entropy will not cause modulo bias.
-            if sample_from_source < sample_max:
-                # Select a replacement index for the current index.
-                replacement_position = (sample_from_source % remaining) + index
-                # Swap the current index with the replacement index.
-                output[index], output[replacement_position] = output[replacement_position], output[index]
-                index += 1
-            else:
-                # The sample causes modulo bias. A new sample should be read.
-                pass
-
-    return output
+        indices = [permute(v) for v in indices]
+    return [v[index] for index in indices]
 ```
 
 ### `split`