Removed macro_rules!{} from packet.rs

[deltronix]

Fixes #57 by:
Replacing $( $lt )* with <'a> in the implementations and removing these blocks around the implementations

macro_rules! impl_* {
  ( $( $lt:tt )* ) => {
     // Various implementations
  }
}

[bruingineer]

This needs further discussion under issue #57