improve compression of undefined, NaN & Infinitiy (#1748)

- migrate transformation logic from `OutputStream` to `Compressor`
- always turn `undefined` into `void 0` (unless `unsafe`)
- always keep `NaN` except when avoiding local variable redefinition
- introduce `keep_infinity` to suppress `1/0` transform, except when avoiding local variable redefinition

supersedes #1723
fixes #1730

test/compress/properties.js

@@ -77,7 +77,7 @@ sub_properties: {
         a[3.14] = 3;
         a.if = 4;
         a["foo bar"] = 5;
-        a[0/0] = 6;
+        a[NaN] = 6;
         a[null] = 7;
         a[void 0] = 8;
     }