Optimize issymbollike

[YingboMa]

julia> @variables x;

julia> @btime issymbollike(Ref($x)[])
  0.026 ns (0 allocations: 0 bytes)
true

julia> @btime issymbollike(Ref(1)[])
  0.026 ns (0 allocations: 0 bytes)
false

julia> @btime issymbollike(Ref(:x)[])
  0.026 ns (0 allocations: 0 bytes)
true

[ChrisRackauckas]

I'm surprised Base.@pure didn't get to this.