String support

[fracek]

When discussing #9 we sort of ignored that implementing tokenUri requires a way to implement strings in Cairo. Strings are also used in ERC-20 (for the optional but so convenient symbol and name methods) and we have seen projects using them to generate dynamic NFT images (returning the NFT as svg).

In discord we mentioned that strings can be implemented by storing bytes in felts (251 bits or 31 bytes per felt). One issue is that, since StarkNet storage cannot store arrays, that's also the maximum string length that can be stored.

[martriay]

Important topic indeed. I'm moving this onto github discussions since I believe this should be implemented at the Cairo level, but keeping the discussion open because I think gathering design input on feature requirements is still valuable.