The only (definitional) criteria of DenseArray is that it's stored contiguously in memory.
We also have Codeunits <: DenseVector. However, there is no requirement that the string backing the codeunits are stored densely in memory. For example, InlineString.jl provide string types which are 8-byte primitives.

I'm not sure what to do about this. It might be breaking to remove Codeunits <: DenseVector, but it's also silly to have it subtype DenseVector when the only thing that matters about DenseVector is not upheld.