WIP: Support List[List[byte, N], N]

[kasey]

Current spectests generated by the methodical repo were not covering ExecutionPayload, so I missed that nested lists were not supported. This commit adds support for List[List[byte, N], N]. 2 things should happen before I remove WIP from the title:

• Cover List[List] via prysm's ExecutionPayload type in spectests.
• double check test coverage of geth's uin256 type and any other types they are currently developing with methodical. Ideally add some additional test fixtures to cover patterns from the existing types.

This PR also has a commit removing all the values/funcs/types flagged by the golangci-lint deadcode checker. There are only 2 commits so it may be easiest to review this PR one commit at a time. I am also happy to split off the lint commit into a separate PR.