Removed memory allocation and copying from SX126X

[BarryPSmith]

Extends command interface to allow multi-byte commands.

This allows functions that need to prepend some parameters onto input data to do so without allocating memory and performing a memcpy.

Benefits:

• SX126x code can now be used without linking in operator new
• Allows transmit and receive to be called when not much memory is available: previously these functions could allocate as much as 257 bytes when dealing with large messages.
• Improved performance by removing memcpy

[jgromes]

Looks good, was this tested on hardware?

[BarryPSmith]

Yes, this is tested.

Aside: I noticed that my pull requests might conflict with each other. If this happens let me know and I can rebase the remaining ones and resolve the conflicts..

[jgromes]

Merged, thanks.

There's no conflict in this one, though there's a minor one in #61