more efficient realdot for arrays

Right now you use `real(LinearAlgebra.dot(x, y))`, which does twice as much work as necessary for complex arrays.

You could do e.g.
```jl
realdot(x::Vector{Complex{T}}, y::Vector{Complex{T}}) where {T} = dot(reinterpret(T, x), reinterpret(T, y))
```
and similar…

Or you could just write out the loop (two `muladd` calls per complex entry, with `@simd`), since BLAS doesn't have much of an advantage for this type of operation.