`AbstractVector` is always considered mutable, even if it's not

[matthias314]

Example:

julia> union(1:1, 2:2)
2-element Vector{Int64}:
 1
 2

julia> union!!(1:1, 2:2)
ERROR: CanonicalIndexError: setindex! not defined for UnitRange{Int64}

The problem seems to be

julia> BangBang.implements(push!, UnitRange)
true

which is a special case of

BangBang.jl/src/core.jl

Line 68 in 9f8c8a5

implements(::Mutator, ::Type{<:MaybeMutableContainer}) = true