resolve circular reference issue when printing

[bkamins]

both text/plain and html

Example:

julia> df = DataFrame(a=Any[nothing])
1×1 DataFrame
 Row │ a
     │ Any
─────┼─────
   1 │

julia> df.a[1] = [df]
1-element Vector{DataFrame}:
Error showing value of type Vector{DataFrame}:
ERROR: StackOverflowError: