title: Indexed variables (symbolic arrays) with ModelingToolkit.jl
body: <p>I would like use ModelingToolkit.jl to solve large nonlinear systems of equations. Unfortunately, using symbolic arrays with NonlinearSystem gives a method error:</p>
  <pre><code>ERROR: MethodError: no method matching hasmetadata(::Vector{Num}, ::Type{Symbolics.VariableDefaultValue})
  </code></pre>
  <p>Is there a way of solving nonlinear equations with indexed variables with ModelingToolkit?</p>
  <p>Code example:</p>
  <pre><code>using ModelingToolkit, NonlinearSolve
  vars = @variables x
  @named works = NonlinearSystem([], vars, [])
  
  vars = @variables x[1:3]
  @named fails = NonlinearSystem([], vars, [])
  
  eqs = [x[j] ~ j for j ∈ 1:3]
  @named also_fails = NonlinearSystem(eqs, vars, [])
  </code></pre>