Add script manipulation tools for use in mininode testing framework

script.py is modified from the code in python-bitcoinlib, and provides tools
for manipulating and creating CScript objects.

bignum.py is a dependency for script.py

script_test.py is an example test that uses the script tools for running a test
that compares the behavior of two nodes, in a comptool- style test, for each of
the test cases in the bitcoin unit test script files, script_valid.json and
script_invalid.json.  (This test is very slow to run, but is a proof of concept
for how we can write tests to compare consensus-critical behavior between
different versions of bitcoind.)

bipdersig-p2p.py is another example test in the comptool framework, which tests
deployment of BIP DERSIG for a single node.  It uses the script.py tools for
manipulating signatures to be non-DER compliant.

qa/rpc-tests/bignum.py

@@ -0,0 +1,102 @@
+#
+#
+# bignum.py
+#
+# This file is copied from python-bitcoinlib.
+#
+# Distributed under the MIT/X11 software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+
+"""Bignum routines"""
+
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import struct
+
+
+# generic big endian MPI format
+
+def bn_bytes(v, have_ext=False):
+    ext = 0
+    if have_ext:
+        ext = 1
+    return ((v.bit_length()+7)//8) + ext
+
+def bn2bin(v):
+    s = bytearray()
+    i = bn_bytes(v)
+    while i > 0:
+        s.append((v >> ((i-1) * 8)) & 0xff)
+        i -= 1
+    return s
+
+def bin2bn(s):
+    l = 0
+    for ch in s:
+        l = (l << 8) | ch
+    return l
+
+def bn2mpi(v):
+    have_ext = False
+    if v.bit_length() > 0:
+        have_ext = (v.bit_length() & 0x07) == 0
+
+    neg = False
+    if v < 0:
+        neg = True
+        v = -v
+
+    s = struct.pack(b">I", bn_bytes(v, have_ext))
+    ext = bytearray()
+    if have_ext:
+        ext.append(0)
+    v_bin = bn2bin(v)
+    if neg:
+        if have_ext:
+            ext[0] |= 0x80
+        else:
+            v_bin[0] |= 0x80
+    return s + ext + v_bin
+
+def mpi2bn(s):
+    if len(s) < 4:
+        return None
+    s_size = bytes(s[:4])
+    v_len = struct.unpack(b">I", s_size)[0]
+    if len(s) != (v_len + 4):
+        return None
+    if v_len == 0:
+        return 0
+
+    v_str = bytearray(s[4:])
+    neg = False
+    i = v_str[0]
+    if i & 0x80:
+        neg = True
+        i &= ~0x80
+        v_str[0] = i
+
+    v = bin2bn(v_str)
+
+    if neg:
+        return -v
+    return v
+
+# bitcoin-specific little endian format, with implicit size
+def mpi2vch(s):
+    r = s[4:]           # strip size
+    r = r[::-1]         # reverse string, converting BE->LE
+    return r
+
+def bn2vch(v):
+    return bytes(mpi2vch(bn2mpi(v)))
+
+def vch2mpi(s):
+    r = struct.pack(b">I", len(s))   # size
+    r += s[::-1]            # reverse string, converting LE->BE
+    return r
+
+def vch2bn(s):
+    return mpi2bn(vch2mpi(s))
+