type level events for bind/result processors

this would allow easy addition of bind and result handlers across existing types and their subclasses without the need for using custom types.
@event.listens_for(String, "bind_processor", retval=True)
def _str_bind_processor(type, dialect):
   if dialect.name == 'oracle':
        def process(value):
            if value is not None:
                value = value.encode(dialect.encoding)
            return value
        return process
   else:
        # type will return default bind processor
        return None



#!diff
diff -r 7c6bde1c15c97cae34ef9449aa595168910fe87d lib/sqlalchemy/types.py
--- a/lib/sqlalchemy/types.py   Thu Oct 25 12:07:38 2012 -0400
+++ b/lib/sqlalchemy/types.py   Thu Oct 25 12:14:41 2012 -0400
@@ -273,7 +273,14 @@
             return dialect._type_memos[self](self)['bind']('bind')
         except KeyError:
             d = self._dialect_info(dialect)
-            d['bind']('bind') = bp = d['impl']('impl').bind_processor(dialect)
+            if self.dispatch.bind_processor:
+                bp = self.dispatch.bind_processor(self, dialect)
+                if bp is None:
+                    bp = d['impl']('impl').bind_processor(dialect)
+            else:
+                bp = d['impl']('impl').bind_processor(dialect)
+
+            d['bind']('bind') = bp
             return bp

     def _cached_result_processor(self, dialect, coltype):
@@ -283,10 +290,18 @@
             return dialect._type_memos[self](self)[coltype](coltype)
         except KeyError:
             d = self._dialect_info(dialect)
+
+            if self.dispatch.result_processor:
+                rp = self.dispatch.result_processor(self, dialect, coltype)
+                if rp is None:
+                    rp = d['impl']('impl').result_processor(dialect, coltype)
+            else:
+                rp = d['impl']('impl').result_processor(dialect, coltype)
+
             # key assumption: DBAPI type codes are
             # constants.  Else this dictionary would
             # grow unbounded.
-            d[coltype](coltype) = rp = d['impl']('impl').result_processor(dialect, coltype)
+            d[coltype](coltype) = rp
             return rp

     def _dialect_info(self, dialect):
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