Draft prototypes for UUIDv6 and beyond. Timestamp-based and machine-sortable.

• UUIDv6 - UUIDv6, aims to be the easiest to implement for applications which already implement UUIDv1.
  The UUIDv6 specification keeps the original Gregorian timestamp source but does not reorder the timestamp bits as per the process utilized by UUIDv1.
  UUIDv6 also requires that pseudo-random data MUST be used in place of the MAC address.
  The rest of the UUIDv1 format remains unchanged in UUIDv6
• UUIDv7 - UUIDv7 introduces an entirely new time-based UUID bit layout utilizing a variable length timestamp sourced from the widely implemented and well known Unix Epoch timestamp source.
  The timestamp is broken into a 36-bit integer sections part, and is followed by a field of variable length which represents the
  sub-second timestamp portion, encoded so that each bit from most to least significant adds more precision
• UUIDv8 - UUIDv8 introduces a relaxed time-based UUID format that caters to application implementations that cannot utilize UUIDv1, UUIDv6, or UUIDv7.
  UUIDv8 also future-proofs this specification by allowing time-based UUID formats from timestamp sources that are not yet be defined.
  The variable size timestamp offers lots of flexibility to create an implementation specific RFC compliant time- based UUID while retaining the properties that make UUID great.

RFC/Draft version implementation: 01
UUIDv6: Not implemented, planned
UUIDv7: Work in progres
UUIDv8: Not planned

Layout and bit order:

bytes      	0               1               2               3
		    0                   1                   2                   3
		    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
		   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		   |                            unixts                             |
		   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
bytes      	4               5               6               7
		   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		   |unixts |       subsec_a        |  ver  |       subsec_b        |
		   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
bytes      	8               9               10              11
		   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		   |var|                   subsec_seq_node                         |
		   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
bytes      	12              13              14              15
		   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		   |                       subsec_seq_node                         |
		   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.22000
AMD Ryzen 7 5800X, 1 CPU, 16 logical and 8 physical cores
.NET SDK=5.0.400
  [Host]   : .NET 5.0.9 (5.0.921.35908), X64 RyuJIT  [AttachedDebugger]
  .NET 5.0 : .NET 5.0.9 (5.0.921.35908), X64 RyuJIT

Job=.NET 5.0  Runtime=.NET 5.0