hush: export -n support

function                                             old     new   delta
builtin_export                                       206     256     +50
set_local_var                                        248     265     +17
expand_variables                                    2204    2203      -1

diff --git a/shell/hush_test/hush-misc/export-n.right b/shell/hush_test/hush-misc/export-n.right
new file mode 100644
index 0000000..3d55bf7
--- /dev/null
+++ b/shell/hush_test/hush-misc/export-n.right
@@ -0,0 +1,10 @@
+export aaa1="'''"
+export aaa2=''
+export aaa3="'''"'abc'
+export aaa8='8'
+aaa9=9
+aaa10=10
+Nothing:
+Nothing:
+Nothing:
+Done