feat: write to any storage slot

[enitrat]

Closes #17
In this example, we demonstrate how we can compute storage addresses onchain with the poseidon hash function, and write to any slot with storage syscalls.

@swapnilraj are there any security issues to consider with this implementation? I think that the output of poseidon is a felt252 so I restricted addresses sizes to 250 bits.

I believe that in the future storage addresses will be computed with Poseidon, so it makes even more sense to use it here.
From there, can use this contract to demonstrate how to create an EternalStorage contract.

[swapnilraj]

I don't see any security considerations from using the poseidon hash and the 250 bit mask. I remember seeing similar Cairo 0 code.
It would be nice to implement the StorageAccess trait for a storage variable and replace its storage to use the Poseidon hash function. This would also be ideal for the EternalStorage pattern UX from the user's perspective if they can interact with their storage variables using normal Cairo syntax but have eternal storage slots for their variables.
Would it be too much to ask for a Map PoC as well?
Happy to see these changes in a following PR.