Avoid sequencesize for more than 2000 statements.

It hardly saves any bytes for a sequence so long, and it risks blowing the
stack with the recursive Seq functions.

Ref #414

lib/compress.js

@@ -479,7 +479,7 @@ merge(Compressor.prototype, {
         };
 
         function sequencesize(statements, compressor) {
-            if (statements.length < 2) return statements;
+            if (statements.length < 2 || statements.length > 2000) return statements;
             var seq = [], ret = [];
             function push_seq() {
                 seq = AST_Seq.from_array(seq);