introducing a local variable that overrides a global handle breaks things

``` julia> using ResumableFunctions julia> f() = 1 f (generic function with 1 method) julia> @resumable function resfun() f = f() @yield 2 end resfun (generic function with 1 method) julia> collect(resfun()) ERROR: UndefRefError: access to undefined reference Stacktrace: [1] getproperty @ ./Base.jl:49 [inlined] [2] ##resfun_FSMI#231 @ ~/Documents/ScratchSpace/quantumjulia/ResumableFunctions.jl/src/macro.jl:2 [inlined] [3] ##resfun_FSMI#231 @ ~/Documents/ScratchSpace/quantumjulia/ResumableFunctions.jl/src/macro.jl:213 [inlined] [4] iterate (repeats 2 times) @ ~/Documents/ScratchSpace/quantumjulia/ResumableFunctions.jl/src/types.jl:24 [inlined] [5] _collect(cont::UnitRange{Int64}, itr::var"##resfun_FSMI#231"{Union{}}, ::Base.HasEltype, isz::Base.SizeUnknown) @ Base ./array.jl:727 [6] collect(itr::var"##resfun_FSMI#231"{Union{}}) @ Base ./array.jl:716 [7] top-level scope @ REPL[4]:1 ``` found by @hanakl