binary: forward LittleEndian and BigEndian vars from encoding/binary

forwards `binary.LittleEndian` and `binary.BigEndian` vars from `encoding/binary`

this is mostly for convenience so users don't have to:
```go
 import  (
  encbin "encoding/binary"
  "github.com/alecthomas/binary"
)
```

just for these 2 vars.