SymbolicUtils seems broken on Julia 1.6

[nathanaelbosch]

(@v1.6) pkg> activate --temp
  Activating new environment at `/tmp/jl_VXJ5AZ/Project.toml`

(jl_VXJ5AZ) pkg> add SymbolicUtils
[...]

then

julia> using SymbolicUtils
[ Info: Precompiling SymbolicUtils [d1185830-fcd6-423d-90d6-eec64667417b]
ERROR: LoadError: LoadError: UndefVarError: nodevalue not defined
Stacktrace:
  [1] getproperty(x::Module, f::Symbol)
    @ Base ./Base.jl:26
  [2] top-level scope
    @ ~/.julia/packages/SymbolicUtils/ssQsQ/src/inspect.jl:4
  [3] include(mod::Module, _path::String)
    @ Base ./Base.jl:384
  [4] include(x::String)
    @ SymbolicUtils ~/.julia/packages/SymbolicUtils/ssQsQ/src/SymbolicUtils.jl:4
  [5] top-level scope
    @ ~/.julia/packages/SymbolicUtils/ssQsQ/src/SymbolicUtils.jl:31
  [6] include
    @ ./Base.jl:384 [inlined]
  [7] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt64}}, source::Nothing)
    @ Base ./loading.jl:1235
  [8] top-level scope
    @ none:1
  [9] eval
    @ ./boot.jl:360 [inlined]
 [10] eval(x::Expr)
    @ Base.MainInclude ./client.jl:446
 [11] top-level scope
    @ none:1
in expression starting at /home/nbosch/.julia/packages/SymbolicUtils/ssQsQ/src/inspect.jl:4
in expression starting at /home/nbosch/.julia/packages/SymbolicUtils/ssQsQ/src/SymbolicUtils.jl:1
ERROR: Failed to precompile SymbolicUtils [d1185830-fcd6-423d-90d6-eec64667417b] to /home/nbosch/.julia/compiled/v1.6/SymbolicUtils/jl_nbH72m.

[nathanaelbosch]

The issue seemed related to SymbolicUtils being compatible with AbstractTrees v0.3 even though it isn't, so with #564 this issue is resolved.