possible to support UInt128?

Hello!

I am trying to `@avx` the following code:
```
function fastmod!(n::Array{UInt32,N},T::Mod) where N
@avx n .= (convert.(UInt128,(T.modulo_inv .* n)) .* T.modulo) .>> 64
end
```
the error I get is

```
MethodError: no method matching UInt128(::VectorizationBase.Vec{8, UInt32})
Closest candidates are:
(::Type{T})(::T) where T<:Number at boot.jl:760
(::Type{T})(::AbstractChar) where T<:Union{AbstractChar, Number} at char.jl:50
(::Type{T})(::BigInt) where T<:Union{UInt128, UInt16, UInt32, UInt64, UInt8} at gmp.jl:347
```
which I assume means that UInt128 conversion is not supported.

It is possible on 64-bit architectures to implement this?

Thanks!