Optimize Next implementation

Once the program detects that we have stepped into another function,
we simply calculate the return address and then set a breakpoint and
continue to that location, avoiding numerous syscalls.

dwarf/frame/table_test.go

@@ -56,7 +56,7 @@ func TestFindReturnAddress(t *testing.T) {
 		syscall.PtracePeekText(p.Pid, uintptr(addr), data)
 		addr = binary.LittleEndian.Uint64(data)
 
-		expected := uint64(0x400f04)
+		expected := uint64(0x400ed4)
 		if addr != expected {
 			t.Fatalf("return address not found correctly, expected %#v got %#v", expected, addr)
 		}