Use turbo-crc32 package to calculate crc32.

[reklatsmasters]

turbo-crc32 is about 2x times faster than crc.

crc:            987.866ms
buffer-crc32:   1387.205ms
turbo-crc32:    447.711ms

[coveralls]

Coverage remained the same at 100.0% when pulling f3ef163 on reklatsmasters:patch/crc32 into 89c048a on koajs:master.

[coveralls]

Coverage remained the same at 100.0% when pulling f3ef163 on reklatsmasters:patch/crc32 into 89c048a on koajs:master.

[kroleg]

@reklatsmasters i was thinking today if it's possible to remove crc completly and simply compare JSON.stringify'ed representations of session before and after. What do you think about it?

[reklatsmasters]

@kroleg I did not completely understand how to do it.

[millette]

@kroleg A stable JSON.stringify would be needed when comparing since the order of keys isn't garanteed. Something like https://github.com/substack/json-stable-stringify although there are faster options.

[kroleg]

@millette i don't think so. All json-stable-stringify does is keys sorting. We don't need it. If session object wasn't modified then JSON.stringifys output will not change.