Derive NFData instances and export VKey and VKeyGenesis patterns as data constructors

[intricate]

• The NFData instances are required for work being done as part of Add Shelley keypair generation cardano-node#799.

• The patterns VKey and VKeyGenesis are exported as data constructors of the type DiscVKey in order to prevent modules that import these symbols from being forced to enable PatternSynonyms.

  i.e., you can do:

  import Shelley.Spec.Ledger.Keys (DiscVKey(VKey, VKeyGenesis))

  instead of:

  import Shelley.Spec.Ledger.Keys (pattern VKey, pattern VKeyGenesis)

• The COMPLETE pragma to indicate that the set of patterns VKey and VKeyGenesis are complete, therefore preventing non-exhaustive pattern match warnings.

[JaredCorduan]

LGTM