Adjust to upcoming Base Vararg change

There's two changes here:
1. Drop unnecessary `<:` in Vararg. Tuples are already covariant and this form will be deprecated.
2. Use Base.unwrapva, rather than open coding the equivalent, since the implementation will change.

Author: Keno

src/traits.jl

@@ -153,9 +153,9 @@ number of dimensions, and their dimensions match as determined by [`dimmatch`](@
 """
 @pure sizematch(::Size{S1}, ::Size{S2}) where {S1, S2} = sizematch(S1, S2)
 @inline sizematch(::Tuple{}, ::Tuple{}) = true
-@inline sizematch(S1::Tuple{Vararg{<:StaticDimension, N}}, S2::Tuple{Vararg{<:StaticDimension, N}}) where {N} =
+@inline sizematch(S1::Tuple{Vararg{StaticDimension, N}}, S2::Tuple{Vararg{StaticDimension, N}}) where {N} =
     dimmatch(S1[1], S2[1]) && sizematch(Base.tail(S1), Base.tail(S2))
-@inline sizematch(::Tuple{Vararg{<:StaticDimension}}, ::Tuple{Vararg{<:StaticDimension}}) = false # mismatch in number of dimensions
+@inline sizematch(::Tuple{Vararg{StaticDimension}}, ::Tuple{Vararg{StaticDimension}}) = false # mismatch in number of dimensions
 
 """
     sizematch(::Size, A::AbstractArray)

src/util.jl

@@ -16,10 +16,7 @@ end
 @generated function promote_tuple_eltype(::Union{T,Type{T}}) where T <: Tuple
     t = Union{}
     for i = 1:length(T.parameters)
-        tmp = T.parameters[i]
-        if tmp <: Vararg
-            tmp = tmp.parameters[1]
-        end
+        tmp = Base.unwrapva(T.parameters[i])
         t = :(promote_type($t, $tmp))
     end
     return quote