* Memory naming tweaks
- Define the channel count and value limbs in a single place, so they're easy to adjust.
- Standardize on "channels" which I think is more explicit, since e.g. `num_mem_ops` used to mean either the channel count or total operation count in a trace.
* feedback
* tweaks
* fmt
It seems redundant in most contexts, e.g. `use plonky2::field::extension_field::Extendable;`. One could import `extension_field`, but it's not that common in Rust, and `field::extension` is now about as short.
