scripts: Adopt __get__ binding for write/writeln methods

This actually binds our custom write/writeln functions as methods to the
file object:

  def writeln(self, s=''):
      self.write(s)
      self.write('\n')
  f.writeln = writeln.__get__(f)

This doesn't really gain us anything, but is a bit more correct and may
be safer if other code messes with the file's internals.

scripts/test.py

@@ -374,17 +374,18 @@ def compile(test_paths, **args):
     # write generated test source
     if 'output' in args:
         with openio(args['output'], 'w') as f:
-            _write = f.write
-            def write(s):
-                f.lineno += s.count('\n')
-                _write(s)
-            def writeln(s=''):
-                f.lineno += s.count('\n') + 1
-                _write(s)
-                _write('\n')
+            # some helpful file functions
             f.lineno = 1
-            f.write = write
-            f.writeln = writeln
+            f.write_ = f.write
+            def write(self, s):
+                self.lineno += s.count('\n')
+                self.write_(s)
+            f.write = write.__get__(f)
+            def writeln(self, s=''):
+                self.lineno += s.count('\n') + 1
+                self.write_(s)
+                self.write_('\n')
+            f.writeln = writeln.__get__(f)
 
             f.writeln("// Generated by %s:" % sys.argv[0])
             f.writeln("//")