allocate the memory of XXHash object on stack

[wanghaao]

In the case of frequently calculating hash values, it performs better.

[OneOfOne]

Can you add a benchmark on why this is needed? and add one for 64bit.

[wanghaao]

Can you add a benchmark on why this is needed? and add one for 64bit.

Thanks for your comments.

I have added the case for 64bit.
It's difficult to add a benchmark. Goland will apply for memory from os in the initial process, then the benchmark-test will reuse this memory space. However, It is beneficial to the business scene where has been existed frequent memory allocation.