Cache member.roles access to avoid surprising performance traps

Without the cache, repeated access could be accidentally quadratic or
worse.

discord/member.py

@@ -161,7 +161,8 @@ class Member(discord.abc.Messageable, _BaseUser):
         Nitro boost on the guild, if available. This could be ``None``.
     """
 
-    __slots__ = ('_roles', 'joined_at', 'premium_since', '_client_status', 'activities', 'guild', 'nick', '_user', '_state')
+    __slots__ = ('_roles', '_cs_roles', 'joined_at', 'premium_since', '_client_status',
+                 'activities', 'guild', 'nick', '_user', '_state')
 
     def __init__(self, *, data, guild, state):
         self._state = state
@@ -233,6 +234,11 @@ class Member(discord.abc.Messageable, _BaseUser):
         self.activities = member.activities
         self._state = member._state
 
+        try:
+            del self._cs_roles
+        except AttributeError:
+            pass
+
         # Reference will not be copied unless necessary by PRESENCE_UPDATE
         # See below
         self._user = member._user
@@ -244,6 +250,10 @@ class Member(discord.abc.Messageable, _BaseUser):
 
     def _update_roles(self, data):
         self._roles = utils.SnowflakeList(map(int, data['roles']))
+        try:
+            del self._cs_roles
+        except AttributeError:
+            pass
 
     def _update(self, data):
         # the nickname change is optional,
@@ -334,7 +344,7 @@ class Member(discord.abc.Messageable, _BaseUser):
         """
         return self.colour
 
-    @property
+    @utils.cached_slot_property('_cs_roles')
     def roles(self):
         """List[:class:`Role`]: A :class:`list` of :class:`Role` that the member belongs to. Note
         that the first element of this list is always the default '@everyone'