SealKeyToTPM should take an array rather than a slice for the key

[chrisccoulson]

The key is supplied to SealKeyToTPM via a byte slice. The first thing that SealKeyToTPM does is check that the supplied key is the correct length (32 bytes), but it would probably be better if the key was supplied as a 32byte array instead ([32]byte).

[cmatsuoka]

The same thing happens in InitializeLUKS2Container.