make Primitives able to support any AbstractVector{UInt8} data

[ExpandingMan]

This should be straightforward but will require lots of changes. The new Primitive definition should be

struct Primitive{J,T<:AbstractVector{UInt8}} <: AbstractPrimitive{J}
    length::Int
    values_idx::Int64
    data::T
end

This is a low priority because as far as I can tell the data should be a Vector{UInt8} in the vast majority of cases. In 0.7 IOBuffers and mmap still use Vector{UInt8} and not a special type.