Reduce memory usage by using __slots__ (#4904)

In some cases this reduces the memory usage of mypy and mypy-daemon
over 30%. This seems to have only a minor effect on speed. Depending on
the workload, this seems to either speed up or slow down runtimes by a
few percent.

Notes:

* Refactor various attribute initializations away from class bodies, as they
  conflict with __slots__.
* This only adds __slots__ to classes that use the most memory, based
  on the output from the memory profiler.
* Some attributes had None defaults even though the type was not
  optional. Now we use a cast for some of these, which has the same
  effect.
* Various __dict__ manipulations had to be modified to support
  __slots__ as well.

Author: JukkaL

mypy/fixup.py

@@ -69,7 +69,7 @@ def visit_symbol_table(self, symtab: SymbolTable) -> None:
         for key, value in list(symtab.items()):
             cross_ref = value.cross_ref
             if cross_ref is not None:  # Fix up cross-reference.
-                del value.cross_ref
+                value.cross_ref = None
                 if cross_ref in self.modules:
                     value.node = self.modules[cross_ref]
                 else:
@@ -152,7 +152,7 @@ def visit_instance(self, inst: Instance) -> None:
         type_ref = inst.type_ref
         if type_ref is None:
             return  # We've already been here.
-        del inst.type_ref
+        inst.type_ref = None
         inst.type = lookup_qualified_typeinfo(self.modules, type_ref, self.quick_and_dirty)
         # TODO: Is this needed or redundant?
         # Also fix up the bases, just in case.