Implement AccessAddress generation

[jonas-schievink]

This is needed if the device wants to initiate a connection (by sending a connection request PDU), it is not needed if the device is just a peripheral that accepts a connection. It is specified in Part B 2.1.2 of Bluetooth Core v4.2.

Depends on #2.

Things that need to be done:

• Create a struct AccessAddress(u32); newtype
• Replace current uses of u32 for access addresses with the new type
  • The current advertising::ACCESS_ADDRESS constant can be replaced by an associated const AccessAddress::ADVERTISING
• Implement Debug by hand and print the hex representation of the address
• Implement a is_valid method that checks the requirements in the specification (also implement a lot of unit tests for this)
• Implement a random constructor method that uses a rand_core::RngCore implementor to generate a random AccessAddress that adheres to the specified requirements