Add a deeply recursive schema/union test

This adds a test from [this pydantic
issue](pydantic/pydantic#8499) to ensure that
we don't error out when processing deeply recursive schemas with unions.
No code change is needed at this time, we process this schema
efficiently already.

tests/test_performance.py

@@ -0,0 +1,51 @@
+import textwrap
+
+import msgspec
+
+from utils import temp_module
+
+
+def test_process_large_recursive_union():
+    """
+    A recursive schema processing perf test from
+    https://github.com/pydantic/pydantic/issues/8499
+
+    This test is mostly to ensure that processing deeply recursive schemas with
+    unions succeeds.
+    """
+
+    def gen_code():
+        yield "from __future__ import annotations"
+        yield "from msgspec import Struct"
+        yield "from typing import Union"
+
+        for i in range(50):
+            yield textwrap.dedent(
+                f"""
+                class Node{i}(Struct, tag='node{i}'):
+                    data: Union[Node, None]
+                """
+            )
+        yield "Node = Union["
+        for i in range(50):
+            yield f"    Node{i},"
+        yield "]"
+
+    code = "\n".join(gen_code())
+
+    with temp_module(code) as mod:
+        dec = msgspec.json.Decoder(mod.Node)
+
+    msg = b"""
+    {
+        "type": "node25",
+        "data": {
+            "type": "node13",
+            "data": null
+        }
+    }
+    """
+
+    sol = mod.Node25(mod.Node13(None))
+
+    assert dec.decode(msg) == sol