printing types of unit quantities loses information

Example:
```
julia> using Unitful: 𝐌,𝐋,L,mg

julia> typeof(1u"mg/L")
Quantity{Int64,𝐌*𝐋^-3,Unitful.FreeUnits{(mg, L^-1),𝐌*𝐋^-3,nothing}}

julia> 1u"mg/L" isa Quantity{Int64,𝐌*𝐋^-3,Unitful.FreeUnits{(mg, L^-1),𝐌*𝐋^-3,nothing}}
false
```
So if I copy and paste the representation of `typeof(1u"mg/L")`, it is not the correct type.

Reproduce the Julia REPL example using Unitful, typeof, Quantity, and FreeUnits, then trace how the type representation is printed. Done means the displayed type for typeof(1u"mg/L") can be copied and used as the equivalent type expression without losing information.

Written by the indexing model from the issue text.