Add UUID v6

[ryanwinchester]

Add UUID v6 (RFC draft) function.

Related web page: http://gh.peabody.io/uuidv6/

I tried to follow the existing code style and conventions as much as possible.

Example reference

For reference (to see how similar a v1 is to a v6), here is a python example from the RFC author converting a UUID v1 to a UUID v6:

import uuid

def uuidv1tov6(u):
  uh = u.hex
  tlo1 = uh[:5]
  tlo2 = uh[5:8]
  tmid = uh[8:12]
  thig = uh[13:16]
  rest = uh[16:]
  uh6 = thig + tmid + tlo1 + '6' + tlo2 + rest
  return uuid.UUID(hex=uh6)

u = uuidv1tov6(uuid.uuid1())

Benchmarks

▶ mix bench
Settings:
  duration:      1.0 s

## UUIDBench
[17:17:54] 1/9: binary_to_string!
[17:18:04] 2/9: info!
[17:18:06] 3/9: string_to_binary!
[17:18:09] 4/9: uuid1
[17:18:11] 5/9: uuid3 dns
[17:18:13] 6/9: uuid4
[17:18:16] 7/9: uuid5 dns
[17:18:17] 8/9: uuid6 mac_address
[17:18:20] 9/9: uuid6 random_bytes

Finished in 27.79 seconds

## UUIDBench
benchmark name      iterations   average time
binary_to_string!     10000000   0.84 µs/op
uuid3 dns              1000000   1.45 µs/op
uuid5 dns              1000000   1.54 µs/op
uuid4                  1000000   2.39 µs/op
string_to_binary!      1000000   3.12 µs/op
info!                   500000   3.13 µs/op
uuid6 random_bytes      500000   4.19 µs/op
uuid6 mac_address        10000   208.53 µs/op
uuid1                    10000   209.47 µs/op

Closes #44

[ryanwinchester]

Added benchmarks