Improve performance by making hashing work on streams

[martinlindhe]

As brought up in #1 (comment), the hashing performance is not great when working on larger files.

The gohash code could be reworked to work on streams in many cases where the underlying hash libraries support it, giving us better performance.

[martinlindhe]

All supported hash algos now work using io.Copy()

[sergeevabc]

All supported hash algos now work using io.Copy()

It did not solve a performance issue described earlier.

[martinlindhe]

All supported hash algos now work using io.Copy()

It did not solve a performance issue described earlier.

Your earlier comment is about bad performance in the gost algorithm.
The implementation in use lives in another repository and are created by other people who are in better position of improving the performance.

This issue is about improving performance in gohash by using i/o streams where possible, which has now been resolved.