Removal of un-checked numerics

[prasannavl]

A tracker to remove all the unchecked numeric ops on the Rust crates

• There are a few unchecked numeric ops left that can cause crashed when certain assumptions of the full pipeline are broken. This issue is to remove all of these entirely and have graceful error propagation up.

• evm: Checked U256 casting #2567

• evm: Use checked operations #2579

[prasannavl]

Safety and miner opt. PRs like #2575 will incidentally cover this up (which was the cause for rc1 crash), but a few more left:

A quick grep after #2567.

rg -g "lib/**/*.rs" '\w* [-+*/] \w*'

Key usages (and likely a few more):

lib/ain-evm/src/executor.rs                                    
205:        if !system_tx && total_gas_used + U256::from(used_gas) > block_gas_limit {

lib/ain-evm/src/evm.rs                                         
182:        let total_burnt_fees = template.total_gas_used * base_fee;
183:        let total_priority_fees = total_gas_fees - total_burnt_fees;
516:            Some(block) => (block.header.number + 1, block.header.state_root),

lib/ain-evm/src/receipt.rs                                     
96:                    logs_index: logs_size - logs_len,

lib/ain-evm/src/weiamount.rs                                   
21:        Ok(self.0 / WEI_TO_GWEI)
30:        Ok(self.0 / WEI_TO_SATS)

lib/ain-grpc/src/rpc/debug.rs                                  
190:                let gas_fee = cmp::min(max_fee_per_gas, max_priority_fee_per_gas + base_fee);
202:        let burnt_fee = used_gas * base_fee;
203:        let priority_fee = gas_fee - burnt_fee;

lib/ain-grpc/src/rpc/debug.rs                                  
190:                let gas_fee = cmp::min(max_fee_per_gas, max_priority_fee_per_gas + base_fee);
202:        let burnt_fee = used_gas * base_fee;
203:        let priority_fee = gas_fee - burnt_fee;