Correctly handle functors when auto-generating column names

[bkamins]

An interesting corner-case fix of:

julia> struct Identity
       end

julia> function (::Identity)(x)
           return x
       end

julia> i = Identity()
Identity()

julia> df = DataFrame(x=1:3)
3×1 DataFrame
 Row │ x
     │ Int64
─────┼───────
   1 │     1
   2 │     2
   3 │     3

julia> combine(df, :x => identity∘i)
ERROR: MethodError: no method matching nameof(::Identity)
Closest candidates are:
  nameof(::Module) at reflection.jl:16
  nameof(::DataType) at reflection.jl:214
  nameof(::UnionAll) at reflection.jl:215
  ...
Stacktrace:
 [1] funname(f::Identity)

[bkamins]

Nightly fails because of JuliaLang/julia#42991.

[nalimilan]

Maybe better use applicable rather than try?

[bkamins]

ok - changed

[bkamins]

Thank you!