Simplify a few conversions to `Expression`

[matthiasgoergens]

Just something that came up while reviewing some other code.

Especially ceno_zkvm/src/chip_handler/memory.rs and ceno_zkvm/src/chip_handler/register.rs become a bit more readable.

[matthiasgoergens]

It's also possible to pack more fields into fewer vec.

Sorry, what do you mean?

[naure]

                [
                    vec![RAMType::Memory.into(), memory_addr.clone()],
                    vec![value.clone()],
                    vec![prev_ts.clone()],
                ]
                .concat()

to

vec![RAMType::Memory.into(), memory_addr.clone(), value.clone(), prev_ts.clone()]

and similar.

[matthiasgoergens]

Oh, I see. Thanks for explaining. I don't even know why our code is so silly in the first place.