support export statements properly (#2126)

- mangle & compress correctly with `toplevel`
- retain non-toplevel import/export
- parse & output `export { variable as name }`
- remove extraneous spaces from `beautify`


fixes #2038
fixes #2124

test/compress/export.js

@@ -0,0 +1,97 @@
+issue_2038_1: {
+    options = {
+        toplevel: true,
+        unused: true,
+    }
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        export var V = 1;
+        export let L = 2;
+        export const C = 3;
+    }
+    expect: {
+        export var V = 1;
+        export let L = 2;
+        export const C = 3;
+    }
+}
+
+issue_2038_2: {
+    options = {
+        toplevel: true,
+        unused: true,
+    }
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        let LET = 1;
+        const CONST = 2;
+        var VAR = 3;
+        export { LET, CONST, VAR };
+    }
+    expect: {
+        let a = 1;
+        const c = 2;
+        var n = 3;
+        export { a as LET, c as CONST, n as VAR };
+    }
+}
+
+issue_2124: {
+    options = {
+        unused: true,
+    }
+    input: {
+        {
+            export var V = 1;
+        }
+        {
+            export let L = 2;
+        }
+        {
+            export const C = 3;
+        }
+    }
+    expect: {
+        {
+            export var V = 1;
+        }
+        {
+            export let L = 2;
+        }
+        {
+            export const C = 3;
+        }
+    }
+}
+
+issue_2126: {
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        import { foo as bar, cat as dog } from "stuff";
+        console.log(bar, dog);
+        export { bar as qux };
+        export { dog };
+    }
+    expect: {
+        import { foo as o, cat as f } from "stuff";
+        console.log(o, f);
+        export { o as qux };
+        export { f as dog };
+    }
+}
+
+beautify: {
+    beautify = {
+        beautify: true,
+    }
+    input: {
+        export { A as B, C as D };
+    }
+    expect_exact: "export { A as B, C as D };"
+}