Keep unused function arguments by default

Discarding unused function arguments affects function.length, which can lead
to some hard to debug issues.  This optimization is now done only in "unsafe
mode".

Fix #121

lib/compress.js

@@ -1086,7 +1086,7 @@ merge(Compressor.prototype, {
             var tt = new TreeTransformer(
                 function before(node, descend, in_list) {
                     if (node instanceof AST_Lambda && !(node instanceof AST_Accessor)) {
-                        if (!compressor.option("keep_fargs")) {
+                        if (compressor.option("unsafe") && !compressor.option("keep_fargs")) {
                             for (var a = node.argnames, i = a.length; --i >= 0;) {
                                 var sym = a[i];
                                 if (sym.unreferenced()) {