Const fns are here

Additions:

• Const Uuid::from_bytes added (#218)
• Backport u128 support (#238)

Dependency Updates:

• serde & serde-test to 1.0.56 (#240)
• cfg_if fixed to min 0.1.2

Feature Gates:

• const_fn feature gate added (#232)

Collaborators:

• @jethrogb
• @mokeyish