benchmark a reasonable size for the buffer

[Razaloc]

In bmap::copy we use a vector as buffer. We need to find the right size to set it up before using it. The current value is 8 * 1024 * 1024. (bmap/src/lib.rs line 46)

[Razaloc]

As an idea this value could be a constant since it's going to be shared by different copy functions. Something like BUFFER_SIZE