A pure-java implementation of the CRC32 checksum that uses
the same polynomial as the built-in native CRC32.
This is to avoid the JNI overhead for certain uses of Checksumming
where many small pieces of data are checksummed in succession.
The current version is ~10x to 1.8x as fast as Sun's native
java.util.zip.CRC32 in Java 1.6
Copied from Hadoop 2.6.3.
The code agrees with the following file in the 2.6.3 tag: