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/numbers.js

@@ -186,7 +186,7 @@ unary_binary_parenthesis: {
         });
     }
     expect: {
-        var v = [ 0, 1, 0/0, 1/0, null, void 0, true, false, "", "foo", /foo/ ];
+        var v = [ 0, 1, NaN, 1/0, null, void 0, true, false, "", "foo", /foo/ ];
         v.forEach(function(x) {
             v.forEach(function(y) {
                 console.log(