testcrypt.py: marshal string literals more efficiently.

The testcrypt protocol expects a string literal to be a concatenation
of literal bytes other than '%' and '\n', and %-escaped hex digit
pairs. But testcrypt.py was only ever using the latter format, so even
a legible ASCII string like "123" was being sent to testcrypt as the
unreadable and needlessly long "%31%32%33".

When debugging, I often arrange to save the testcrypt input stream to
a file, and sometimes I use that file as the starting point for
editing. So it is actually useful to have the protocol exchange be
legible to humans. Hence, here's a change to testcrypt.py which makes
it only use the %-escape encoding for byte values that aren't
printable ASCII.

test/testcrypt.py

@@ -140,6 +140,14 @@ class Value(object):
     def __int__(self):
         return int(self.__long__())
 
+def marshal_string(val):
+    val = coerce_to_bytes(val)
+    assert isinstance(val, bytes), "Bad type for val_string input"
+    return "".join(
+        chr(b) if (0x20 <= b < 0x7F and b != 0x25)
+        else "%{:02x}".format(b)
+        for b in val)
+
 def make_argword(arg, argtype, fnname, argindex, to_preserve):
     typename, consumed = argtype
     if typename.startswith("opt_"):
@@ -147,12 +155,9 @@ def make_argword(arg, argtype, fnname, argindex, to_preserve):
             return "NULL"
         typename = typename[4:]
     if typename == "val_string":
-        arg = coerce_to_bytes(arg)
+        retwords = childprocess.funcall("newstring", [marshal_string(arg)])
-        if isinstance(arg, bytes):
+        arg = make_retvals([typename], retwords, unpack_strings=False)[0]
-            retwords = childprocess.funcall(
+        to_preserve.append(arg)
-                "newstring", ["".join("%{:02x}".format(b) for b in arg)])
-            arg = make_retvals([typename], retwords, unpack_strings=False)[0]
-            to_preserve.append(arg)
     if typename == "val_mpint" and isinstance(arg, numbers.Integral):
         retwords = childprocess.funcall("mp_literal", ["0x{:x}".format(arg)])
         arg = make_retvals([typename], retwords)[0]