Macro buf_get_impl

macro_rules! buf_get_impl {
    ($this:ident, $typ:tt::$conv:tt) => { ... };
    (le => $this:ident, $typ:tt, $len_to_read:expr) => { ... };
    (be => $this:ident, $typ:tt, $len_to_read:expr) => { ... };
}