Add valtype(...) for arrays

Arrays are associative structures in the sense that keys(...)
and values(...) both work on them. When considering them as such, their
valtype should be equal to eltype. (A future version of Julia could
even do `const valtype = eltype`?)

base/abstractarray.jl

@@ -107,6 +107,9 @@ keytype(a::AbstractVector) = Int
 keytype(A::Type{<:AbstractArray}) = CartesianIndex{ndims(A)}
 keytype(A::Type{<:AbstractVector}) = Int
 
+valtype(a::AbstractArray) = eltype(a)
+valtype(A::Type{<:AbstractArray}) = eltype(A)
+
 prevind(::AbstractArray, i::Integer) = Int(i)-1
 nextind(::AbstractArray, i::Integer) = Int(i)+1