`@fgenerator` doesn't play nicely with argument destructuring

[simeonschaub]

The following currently throws an error:

julia> struct B x end

julia> @fgenerator((; x)::B) do
           @yield x
       end
ERROR: LoadError: MethodError: no method matching length(::Expr)
Closest candidates are:
  length(::Union{Base.KeySet, Base.ValueIterator}) at ~/packages/julias/julia-1.7/share/julia/base/abstractdict.jl:58
  length(::Union{LinearAlgebra.Adjoint{T, S}, LinearAlgebra.Transpose{T, S}} where {T, S}) at ~/packages/julias/julia-1.7/share/julia/stdlib/v1.7/LinearAlgebra/src/adjtrans.jl:171
  length(::VSCodeServer.JSON.Parser.PushVector) at ~/.vscode/extensions/julialang.language-julia-1.4.3/scripts/packages/JSON/src/pushvector.jl:13
  ...
Stacktrace:
  [1] length(g::Base.Generator{Expr, FGenerators.var"#10#13"})
    @ Base ./generator.jl:50
  [2] _similar_shape(itr::Base.Generator{Expr, FGenerators.var"#10#13"}, #unused#::Base.HasLength)
    @ Base ./array.jl:600
  [3] collect(itr::Base.Generator{Expr, FGenerators.var"#10#13"})
    @ Base ./array.jl:723
  [4] define_foldl(__module__::Module, funcname::Expr, structname::Symbol, allargs::Expr, body::Expr)
    @ FGenerators ~/.julia/packages/FGenerators/N3bhz/src/FGenerators.jl:285
  [5] var"@fgenerator"(__source__::LineNumberNode, __module__::Module, fun::Any, coll::Any)
    @ FGenerators ~/.julia/packages/FGenerators/N3bhz/src/FGenerators.jl:245
  [6] eval
    @ ./boot.jl:373 [inlined]
  [7] eval
    @ ./Base.jl:68 [inlined]
  [8] repleval(m::Module, code::Expr, #unused#::String)
    @ VSCodeServer ~/.vscode/extensions/julialang.language-julia-1.4.3/scripts/packages/VSCodeServer/src/repl.jl:157
  [9] (::VSCodeServer.var"#71#73"{Module, Expr, REPL.LineEditREPL, REPL.LineEdit.Prompt})()
    @ VSCodeServer ~/.vscode/extensions/julialang.language-julia-1.4.3/scripts/packages/VSCodeServer/src/repl.jl:123
 [10] with_logstate(f::Function, logstate::Any)
    @ Base.CoreLogging ./logging.jl:511
 [11] with_logger
    @ ./logging.jl:623 [inlined]
 [12] (::VSCodeServer.var"#70#72"{Module, Expr, REPL.LineEditREPL, REPL.LineEdit.Prompt})()
    @ VSCodeServer ~/.vscode/extensions/julialang.language-julia-1.4.3/scripts/packages/VSCodeServer/src/repl.jl:124
 [13] #invokelatest#2
    @ ./essentials.jl:716 [inlined]
 [14] invokelatest(::Any)
    @ Base ./essentials.jl:714
 [15] macro expansion
    @ ~/.vscode/extensions/julialang.language-julia-1.4.3/scripts/packages/VSCodeServer/src/eval.jl:34 [inlined]
 [16] (::VSCodeServer.var"#55#56")()
    @ VSCodeServer ./task.jl:411
in expression starting at REPL[6]:1

So it seems like the @fgenerator assumes the argument is always a symbol.