make the file save more faster on large files

It improves the saving speed by using crc32 instead of md5 and also disables the "delete white spaces" on large files.

this PR doesn't solve #3028

review @devoncarew 
Let's move this constant to the top of the file (after the export line). Also, I think something like 500,000 would be a better value. spark.dart is currently 128,652. I think the real problem files are the machine generated ones.