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/conditionals.js

@@ -840,8 +840,8 @@ equality_conditionals_false: {
         f(0, true, 0),
         f(1, 2, 3),
         f(1, null, 3),
-        f(0/0),
-        f(0/0, "foo");
+        f(NaN),
+        f(NaN, "foo");
     }
     expect_stdout: true
 }
@@ -888,8 +888,8 @@ equality_conditionals_true: {
         f(0, true, 0),
         f(1, 2, 3),
         f(1, null, 3),
-        f(0/0),
-        f(0/0, "foo");
+        f(NaN),
+        f(NaN, "foo");
     }
     expect_stdout: true
 }