Seebs/decopy
This is a quick patch to allow instrumenting copies of de objects without imposing any runtime costs (in theory) when it's not in use (the functions defined in nop_inst.go can all be inlined, so the compiler will drop them since they do nothing, and won't need to compute their arguments, and the empty embedded structs don't take up space), also a simple benchmark using it. Then there's a second patch which uses this to justify changing the way overflow is handled. The actual substantive change isn't my work, it's from Travis Turner at Pilosa.