Confusing printing of types

```
julia> using Unitful: FreeUnits, s, 𝐓

julia> typeof(s)
FreeUnits{(s,), 𝐓, nothing}

julia> s isa FreeUnits{(s,), 𝐓, nothing}
false
```

I guess there's a distinction between `FreeUnits` and `Unit`, but they both print the same way.

Start by reproducing the Julia REPL example from the issue and inspect how FreeUnits and Unit are displayed. The work is done when their printed representations clearly distinguish the two types and the example no longer appears contradictory.

Written by the indexing model from the issue text.