Make macro context NS readonly

Macros really should not be writing information to their expansion ns that might be used at run time, since run time might not be in the same Python session as compile time.

src/hissp/compiler.py

@@ -17,7 +17,7 @@
 from itertools import chain, starmap, takewhile
 from pprint import pformat
 from traceback import format_exc
-from types import ModuleType
+from types import MappingProxyType, ModuleType
 from typing import Any, Dict, Iterable, List, NewType, Tuple, TypeVar, Union
 from warnings import warn
 
@@ -42,7 +42,7 @@
 @contextmanager
 def macro_context(ns):
     """Sets `NS` during macroexpansions."""
-    token = NS.set(ns)
+    token = NS.set(MappingProxyType(ns))
     try:
         yield
     finally: