Currently, a Layout (UInt n r c) is a Vector (NumberOfRegisters (BaseField c) n r), which is a can of worms:

1. To work with low and high registers, we need to push/pop them from the vector, which is suboptimal;
2. Most operations end up requiring KnownNat (NumberOfRegisters (BaseField c) n r) which clutter the constraints;
3. All in all, we need to compute NumberOfRegisters (BaseField c) n r, which leads to slow compilation and unreadable compilation errors.

All of this can be solved with a custom Layout (UInt n r c) with a custom Representable instance which would only require Symbolic c, KnownNat n and KnownRegisterSize r, which is much more tractable.

P.S. Note that we do not need to make a custom SymbolicData instance for UInt, it's the functor which is used inside that has to be changed from Vector to something else.