Is crc32 realy needed?
Currently to check if session changed `koa-session` does these:
1. calculates crc of JSON.stringify'ed session data when it reads session from cookie/external storage
https://github.com/koajs/session/blob/1600aabdfa6a86973e3fab9f4064c3ed82b10604/lib/context.js#L150
https://github.com/koajs/session/blob/1600aabdfa6a86973e3fab9f4064c3ed82b10604/lib/context.js#L98
https://github.com/koajs/session/blob/1600aabdfa6a86973e3fab9f4064c3ed82b10604/lib/util.js#L35
2. it calculates crc of session before commiting it and then compares new sum with old one.

So if instead of crc as a hash we will use result of JSON.stringify (i.e. `const hash = (sess) => JSON.stringify(sess)`) we can shave of some time from request/response processing. Not much, but it will be -1 dependency. Not sure if memory consumption will increase, i think it depends on garbage collection.

Resolves #131 and #156 (no need in faster crc calculator) 