[V3/permissions] Performance improvements (#1885)

* basic caching layer

* bit more work, now with an upper size to the cache

* cache fix

* smarter cache invalidation

* One more cache case

* Put in a bare skeleton of something else still needed

* more logic handling improvements

* more work, still not finished

* mass-resolve is done in theory, but needs testing

* small bugfixin + comments

* add note about before/after hooks

* LRU-dict fix

* when making comments about optimizations, provide historical context

* fmt pass

redbot/core/utils/caching.py

@@ -0,0 +1,53 @@
+import collections
+
+
+class LRUDict:
+    """
+    dict with LRU-eviction and max-size
+
+    This is intended for caching, it may not behave how you want otherwise
+
+    This uses collections.OrderedDict under the hood, but does not directly expose
+    all of it's methods (intentional)
+    """
+
+    def __init__(self, *keyval_pairs, size):
+        self.size = size
+        self._dict = collections.OrderedDict(*keyval_pairs)
+
+    def __contains__(self, key):
+        if key in self._dict:
+            self._dict.move_to_end(key, last=True)
+            return True
+        return False
+
+    def __getitem__(self, key):
+        ret = self._dict.__getitem__(key)
+        self._dict.move_to_end(key, last=True)
+        return ret
+
+    def __setitem__(self, key, value):
+        if key in self._dict:
+            self._dict.move_to_end(key, last=True)
+        self._dict[key] = value
+        if len(self._dict) > self.size:
+            self._dict.popitem(last=False)
+
+    def __delitem__(self, key):
+        return self._dict.__delitem__(key)
+
+    def clear(self):
+        return self._dict.clear()
+
+    def pop(self, key):
+        return self._dict.pop(key)
+
+    # all of the below access all of the items, and therefore shouldnt modify the ordering for eviction
+    def keys(self):
+        return self._dict.keys()
+
+    def items(self):
+        return self._dict.items()
+
+    def values(self):
+        return self._dict.values()