Bug report: combinations does not work for Sets

to my full surprise the `combinations` function cannot deal with Sets directly.

As Sets support `iterate` and `length`, I think this is a bug
```
julia> collect(combinations(Set([1,2,3,4]), 2))
ERROR: MethodError: no method matching getindex(::Set{Int64}, ::Int64)
Stacktrace:
[1] (::Combinatorics.var"#9#12"{Set{Int64}})(ci::Int64)
@ Combinatorics ./none:0
[2] iterate
@ ./generator.jl:47 [inlined]
[3] collect
@ ./array.jl:724 [inlined]
[4] reorder
@ ~/.julia/packages/Combinatorics/Udg6X/src/combinations.jl:48 [inlined]
[5] #10
@ ./none:0 [inlined]
[6] iterate
@ ./generator.jl:47 [inlined]
[7] collect(itr::Base.Generator{Combinatorics.Combinations, Combinatorics.var"#10#13"{Combinatorics.var"#reorder#11"{Set{Int64}}}})
@ Base ./array.jl:724
[8] top-level scope
@ REPL[16]:1
```

Using Combinatorics v1.0.2 on Linux Julia 1.7.1