fix export related issues (#2005)

- `mangle` non-exported names
- `unused` on `export` of `function`
- `hoist_funs` on `export`
- `export default`
  - prohibit definition statements
  - parse `AST_Defun` properly
  - drop only unused class and function names


fixes #2001
fixes #2004

lib/parse.js

@@ -2417,14 +2417,26 @@ function parse($TEXT, options) {
             }
         }
 
-        var is_definition = is("keyword", "var")
-            || is("keyword", "let")
-            || is("keyword", "const")
-            || is("keyword", "function") && !is_default;
+        var is_definition = is("keyword", "var") || is("keyword", "let") || is("keyword", "const");
         if (is_definition) {
+            if (is_default) unexpected();
             exported_definition = statement();
+        } else if (is("keyword", "class")) {
+            var cls = expr_atom(false);
+            if (cls.name) {
+                cls.name = new AST_SymbolDefClass(cls.name);
+                exported_definition = new AST_DefClass(cls);
+            } else {
+                exported_value = cls;
+            }
         } else if (is("keyword", "function")) {
-            exported_value = expr_atom(false);
+            var func = expr_atom(false);
+            if (func.name) {
+                func.name = new AST_SymbolDefun(func.name);
+                exported_definition = new AST_Defun(func);
+            } else {
+                exported_value = func;
+            }
         } else {
             exported_value = expression(false);
             semicolon();