Verify `layertypes.go`

[jcp19]

PR #76 introduces specification for the file slayers/layertypes.go to allow progress in verification of package slayers. We should switch verify the file once the rest of slayers has been verified.

This may require some fixes in Gobra to allow assigning a closure instance to an interface

Related tasks:

• issue Verify decodeSCION in slayers/scion.go #148
• verify decodeEndToEndExtn and decodeHopByHopExtn in extn.go