Clang didn't accept the friend declaration here, as the class has a
requires clause attached to it, and I couldn't immediately figure
out what it wants instead.
Add accessors for VectorN::m_data and use those where needed instead
for now.
`Gfx::Vector[2,3,4]` are nearly identical implementations. This code
redundancy does not follow the DRY (Don't Repeat Yourself) principle
leading to possible out-of-sync errors between the classes.
Combining these classes into a class template which can be specialized
for each needed size makes the differences obvious through
`constexpr-if` blocks and `requires` clauses.
