Use POSIX I/O for view call proofs

[nategraf]

Currently, the view call proof library uses stdin and env::read to get it's required inputs, namely the Merkle inclusion proofs for the storage slots accessed in view call execution. This introduces from devex complications in that the developer needs to redefine the inputs to include this data.

By using POSIX I/O, this communication can be done out of band to user execution, and the details can be made internal to the view call library itself.

Depends on risc0/risc0-foundry-template#91