Make macroexpand1 crash if macro isn't callable.

Unless it's None, which will be interpreted as no macro by that name being present.
This is more consistent with compiler behavior.
The check should also be a little bit faster.

src/hissp/compiler.py

@@ -609,9 +609,9 @@ def macroexpand1(form, ns=None):
     if type(form) is not tuple or not form:
         return form
     head, *tail = form
-    if callable(macro := Compiler.get_macro(form[0], ns)):
-        return macro(*tail)
-    return form
+    if (macro := Compiler.get_macro(form[0], ns)) is None:
+        return form
+    return macro(*tail)
 
 
 def macroexpand(form, ns=None):