perf: collisions don't contain reserved names by default (#684)

The 'collisions' set does NOT contain RESERVED_NAMES; they are
combined at runtime when needed.
    
For a large real-world, this results in an order of magnitude
reduction in memory usage.
I'm not joking: Google Ads v5 uses 2.45 GB peak before this change
and 223 MB after.

Also contains changes to add `__slots__` attributes to Metadata and Address. These are ancillary, optional, and open to negotiation.
In the above scenario, they reduce memory usage from 223 MB to 177 MB. If other people feel that the reduction in readability does not warrant the reduction in memory usage I am absolutely open to dropping that particular commit.

Includes other minor memory usage optimizations that collectively shave about 5 MB.

gapic/schema/metadata.py

@@ -49,9 +49,6 @@ class Address:
     )
     collisions: FrozenSet[str] = dataclasses.field(default_factory=frozenset)
 
-    def __post_init__(self):
-        super().__setattr__("collisions", self.collisions | RESERVED_NAMES)
-
     def __eq__(self, other) -> bool:
         return all([getattr(self, i) == getattr(other, i) for i
                     in ('name', 'module', 'module_path', 'package', 'parent')])
@@ -114,7 +111,7 @@ def module_alias(self) -> str:
         while still providing names that are fundamentally readable
         to users (albeit looking auto-generated).
         """
-        if self.module in self.collisions:
+        if self.module in self.collisions | RESERVED_NAMES:
             return '_'.join(
                 (
                     ''.join(
@@ -283,7 +280,7 @@ def with_context(self, *, collisions: FrozenSet[str]) -> 'Address':
         ``Address`` object aliases module names to avoid naming collisions in
         the file being written.
         """
-        return dataclasses.replace(self, collisions=frozenset(collisions))
+        return dataclasses.replace(self, collisions=collisions)
 
 
 @dataclasses.dataclass(frozen=True)

gapic/schema/wrappers.py

@@ -314,13 +314,19 @@ def recursive_field_types(self) -> Sequence[
         return tuple(types)
 
     @utils.cached_property
-    def recursive_fields(self) -> FrozenSet[Field]:
-        return frozenset(chain(
+    def recursive_resource_fields(self) -> FrozenSet[Field]:
+        all_fields = chain(
             self.fields.values(),
             (field
              for t in self.recursive_field_types if isinstance(t, MessageType)
              for field in t.fields.values()),
-        ))
+        )
+        return frozenset(
+            f
+            for f in all_fields
+            if (f.options.Extensions[resource_pb2.resource_reference].type or
+                f.options.Extensions[resource_pb2.resource_reference].child_type)
+        )
 
     @property
     def map(self) -> bool:
@@ -1060,7 +1066,7 @@ def gen_resources(message):
                     yield type_
 
         def gen_indirect_resources_used(message):
-            for field in message.recursive_fields:
+            for field in message.recursive_resource_fields:
                 resource = field.options.Extensions[
                     resource_pb2.resource_reference]
                 resource_type = resource.type or resource.child_type