Warn / disallow gigantic storage arrays

[chriseth]

As explained in
https://chriseth.github.io/notes/talks/safe_solidity/#/8
it can be dangerous to mix gigantic arrays in memory with mappings or other data structures. We should either disallow such arrays or warn about them. A complication is the fact that it might be possible to create a multi-dimensional array where each dimension stays below the safety limit, but the full array is again dangerous.