Add a GDB Python script to pretty-print Bignum.

I've been playing around with GDB's Python scripting system recently,
and this is a thing I've always thought it would be nice to be able to
do: if you load this script (which, on Ubuntu 18.04's gdb, is as
simple as 'source contrib/gdb.py' at the gdb prompt, or similar), then
variables of type Bignum will be printed as (e.g.) 'Bignum(0x12345)',
or 'Bignum(NULL)' if they're null pointers, or a fallback
representation if they're non-null pointers but gdb can't read
anything sensible from them.

contrib/gdb.py

@@ -0,0 +1,37 @@
+import gdb
+import re
+import gdb.printing
+
+class PuTTYBignumPrettyPrinter(gdb.printing.PrettyPrinter):
+    "Pretty-print PuTTY's Bignum type."
+    name = "Bignum"
+
+    def __init__(self, val):
+        super(PuTTYBignumPrettyPrinter, self).__init__(self.name)
+        self.val = val
+
+    def to_string(self):
+        type_BignumInt = gdb.lookup_type("BignumInt")
+        type_BignumIntPtr = type_BignumInt.pointer()
+        BIGNUM_INT_BITS = 8 * type_BignumInt.sizeof
+        array = self.val.cast(type_BignumIntPtr)
+        aget = lambda i: int(array[i]) & ((1 << BIGNUM_INT_BITS)-1)
+
+        try:
+            length = aget(0)
+            value = 0
+            for i in range(length):
+                value |= aget(i+1) << (BIGNUM_INT_BITS * i)
+            return "Bignum({:#x})".format(value)
+
+        except gdb.MemoryError:
+            address = int(array)
+            if address == 0:
+                return "Bignum(NULL)".format(address)
+            return "Bignum(invalid @ {:#x})".format(address)
+
+rcpp = gdb.printing.RegexpCollectionPrettyPrinter("PuTTY")
+rcpp.add_printer(PuTTYBignumPrettyPrinter.name, "^Bignum$",
+                 PuTTYBignumPrettyPrinter)
+
+gdb.printing.register_pretty_printer(None, rcpp)