Handle Rent calculations

[dkcumming]

All proof harnesses that have Rent (test_process_{initialize_mint_freeze, initialize_mint_no_freeze, initialize_mint_freeze2, initialize_mint_no_freeze2 initialize_account, initialize_account2, initialize_account3, initialize_multisig, initialize_multisig2, withdraw_excess_lamports}) will all encounter arithmetic overflows and f64 arithmetic (which we do not support currently).

After discussing with Febo, he said that we can assume the overflows are guaranteed impossible by the solana runtime environment. And that the float arithmetic will be removed fully in the future, and the f64 will be a [u8; 8] byte array instead.

I am not sure if we need to intercept the calls with overflow (perhaps they are blocking, I think this needs to be investigated?), but if they are not blocking perhaps they can be left there and we note them as safe to ignore for the reasons above.

[ehildenb]

Maybe we can implement this as "every floating point operation returns 0.0", or something similar, so we can just get rid of thes ebranches. But we need to make sure it doesn't affect other branches that are actually reasonable as well.