Add general UUID / GUID support

[mikee47]

This PR moves the Uuid object out of the SSDP library and makes some improvements.

UUIDs/GUIDs are not exclusively network entities. They're required for filesystems, for example.

• Add template constructor for arbitrary 16-byte entities
• Add comparison operators
• Add Uuid test module
• Fix test message truncation by updating SmingTest to write messages directly to Serial

Todo:

• Replace template constructor as UUID version (endianness) must be considered

[slaff]

@mikee47 are you ready with this PR?

[mikee47]

@slaff Yes thanks, all done for now :-)