Copy in compile_atom instead of macroexpand_1

This copying is what keeps all the mutating code in the compiler methods (e.g., `expr.pop(0)`) from breaking cases in which Hy model objects are compiled more than once or inspected after compilation.

hy/compiler.py

@@ -423,6 +423,8 @@ class HyASTCompiler(object):
             # pass in `atom_type`.
             atom_compiler = _compile_table[atom_type]
             arity = hy.inspect.get_arity(atom_compiler)
+            # Compliation methods may mutate the atom, so copy it first.
+            atom = copy.copy(atom)
             ret = (atom_compiler(self, atom, atom_type)
                    if arity == 3
                    else atom_compiler(self, atom))

hy/macros.py

@@ -181,8 +181,6 @@ def macroexpand_1(tree, compiler):
         fn = tree[0]
         if fn in ("quote", "quasiquote"):
             return tree
-        ntree = HyExpression(tree[:])
-        ntree.replace(tree)
 
         opts = {}
 
@@ -197,14 +195,14 @@ def macroexpand_1(tree, compiler):
 
                 try:
                     m_copy = make_empty_fn_copy(m)
-                    m_copy(compiler.module_name, *ntree[1:], **opts)
+                    m_copy(compiler.module_name, *tree[1:], **opts)
                 except TypeError as e:
                     msg = "expanding `" + str(tree[0]) + "': "
                     msg += str(e).replace("<lambda>()", "", 1).strip()
                     raise HyMacroExpansionError(tree, msg)
 
                 try:
-                    obj = m(compiler.module_name, *ntree[1:], **opts)
+                    obj = m(compiler.module_name, *tree[1:], **opts)
                 except HyTypeError as e:
                     if e.expression is None:
                         e.expression = tree
@@ -214,7 +212,6 @@ def macroexpand_1(tree, compiler):
                     raise HyMacroExpansionError(tree, msg)
                 replace_hy_obj(obj, tree)
                 return obj
-        return ntree
     return tree