Define promotion rules

[oxinabox]

if one e.g. vcats two arrays of ShortStrings the result should be an array of ShortStrings of the larger size.

Like for the integers that back them:

julia> vcat(Int16[1,2,3], Int64[1,2,3])
6-element Vector{Int64}:
 1
 2
 3                                                                                                                                                     1
 2
 3

We want the same for ShortStrings

julia> vcat(ShortString3["ab", "cd"], ShortString7["abcd", "efgc"])
ERROR: MethodError: promote_rule(::Type{Union{}}, ::Type{UInt64}) is ambiguous. Candidates:
  promote_rule(::Type{var"#s818"} where var"#s818"<:AbstractIrrational, ::Type{T}) where T<:Real in Base at irrationals.jl:42
  promote_rule(::Type{S}, ::Type{T}) where {S<:AbstractIrrational, T<:Number} in Base at irrationals.jl:43
  promote_rule(::Type{X}, ::Type{Y}) where {X<:Union{BitIntegers.AbstractBitSigned, BitIntegers.AbstractBitUnsigned}, Y<:Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8, BitIntegers.AbstractBitSigned, BitIntegers.AbstractBitUnsigned}} in BitIntegers at /Users/oxinabox/.julia/packages/BitIntegers/fcpdN/src/BitIntegers.jl:223
Possible fix, define
  promote_rule(::Type{Union{}}, ::Type{Y}) where Y<:Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8, BitIntegers.AbstractBitSigned, BitIntegers.AbstractBitUnsigned}
Stacktrace:
 [1] promote_rule(#unused#::Type{ShortString{Union{}}}, #unused#::Type{ShortString7})
   @ ShortStrings ~/.julia/packages/ShortStrings/qiyr7/src/base.jl:67
 [2] promote_type
   @ ./promotion.jl:233 [inlined]
 [3] promote_result
   @ ./promotion.jl:247 [inlined]
 [4] promote_type
   @ ./promotion.jl:233 [inlined]
 [5] promote_eltype(v1::Vector{ShortString3}, vs::Vector{ShortString7})
   @ Base /usr/local/src/julia/julia-master/base/abstractarray.jl:1413
 [6] vcat(::Vector{ShortString3}, ::Vector{ShortString7})
   @ SparseArrays /usr/local/src/julia/julia-master/usr/share/julia/stdlib/v1.6/SparseArrays/src/sparsevector.jl:1113
 [7] top-level scope
   @ REPL[19]:1

Behavour is correct for vcating Vector{String} to Vector{ShortSting3} though. That does promote to String

julia> vcat(ShortString3["ab", "cd"], ["abcd", "efgc"])
4-element Vector{String}:
 "ab"
 "cd"
 "abcd"
 "efgc"

This requires #18 to be efficient

[xiaodaigh]

ShortString{Union{}}

what I don't understand is , where does this come from?