Fix compilation of certain regexps

tests/objects/test_regexpobject.py

@@ -49,6 +49,7 @@ def test_compile_regexps(self, space):
         /(?:a|b)*/
         /.*/m
         /(.*|.+)/
+        /(?<=b)/
         """)
 
     def test_regexp_syntax_errors(self, space):

topaz/utils/regexp.py

@@ -285,6 +285,9 @@ def __init__(self, value, case_insensitive=False, positive=True, zerowidth=False
     def rebuild(self, positive, case_insensitive, zerowidth):
         return Character(self.value, positive=positive, case_insensitive=case_insensitive, zerowidth=zerowidth)
 
+    def getwidth(self):
+        return 1, 1
+
     def fix_groups(self):
         pass