* Have add_gate take a generic type instead of GateRef
There are a couple advantages
- Users writing their own gates won't need to know about the `GateRef` wrapper; it's more of an internal thing now.
- Easier access to gate methods requiring `self` -- for example, `split_le_base` can just call `gate_type.limbs()` now.
* Update comment
* Always insert
As in plonky1. The semantics of virtual targets in plonky1 were rather weird, but I think it's somewhat better here, since we already separate `generate_copy` and `assert_equal` methods. Users now make more of an explicit choice -- they can use a `VirtualTarget` for the witness generation only using `generate_copy`, or they can involve it in copy constraints.
* Draw challenge points from the extension field
* Now building
* Misc
* Default eval_unfiltered_base
* fmt
* A few field settings
* Add to Sage
* Display tweak
* eval_filtered_base
* Quartic in bench
* Missing methods
* Fix tests
* PR feedback
