Enforce little endian encoding

[jean-roland]

Until know, zenoh-pico assumed the system was little endian which will cause issues if this assumption is wrong.

This PR adds a layer to take into account system endianness and enforce little endian encoding which is the Zenoh encoding. It will solve #420.

• Add an endianness module.
• Use this module on the api/slice side.
• Use the module on the protocole / wire encoding side.