random: Add randrange() implementation.

The idea behind this implementation is that getrandbits() is guaranteed
(required) to be equally distributed. Thus, we can just repeatedly
sample it until get a suitable value, there's no bias accumulated and
the process should be finite (and on average take few iterations).

random/random.py

@@ -0,0 +1,18 @@
+from urandom import *
+
+
+def randrange(start, stop=None):
+    if stop is None:
+        stop = start
+        start = 0
+    upper = stop - start
+    bits = 0
+    pwr2 = 1
+    while upper > pwr2:
+        pwr2 <<= 1
+        bits += 1
+    while True:
+        r = getrandbits(bits)
+        if r < upper:
+            break
+    return r + start