a source <code>src</code> to a destination <code>dst</code> and returns the
number of elements copied.
The <a href="#Core_types">core types</a> of both arguments must be slices
with <a href="#Type_identity">identical</a> element type.
The number of elements copied is the minimum of
<code>len(src)</code> and <code>len(dst)</code>.
As a special case, if the destination's core type is <code>[]byte</code>,