Simplify bwtraceboundary function signature

[hollandjg]

          Because both `UInt8` and `Int64` are subtypes of `Integer`, the function signature could be simplified to something like

function foo(
    image::AbstractArray{T,2};
    P0::Union{Tuple{Integer,Integer},CartesianIndex{2},Nothing}=nothing,
    closed::Bool=true,
) where {T<:Union{Integer,AbstractFloat,Bool}}

Originally posted by @cpaniaguam in #458 (comment)