text and implicit-only-string

Trying to convert `pack (show taxTypeRate) <> "% " <> taxTypeName ` using implicit-only-string. `taxTypeName` and the result is a `Text`. The obvious conversiot

``` haskell
"${taxTypeRate}%${taxTypeName}
```

Does not work out of the box; the only way I found to make it work is to use `unpack` (either explicitely or instanciating

``` haskell
instance Interpolate Text where
interpolate = unpack
```

In both the cases the codes result in somethingsimilar to

``` haksell
mconcat [ fromString (show taxTypeRate
, fromString "% "
, fromString (unpack taxTypeName)
]
```

This `fromString (unpack ...)` seems inefficient, unless there is a rule converting `fromString . upnack` to `id` ???

No files or tests are named. Start by reproducing the Text interpolation example from the issue and inspect the Interpolate Text instance and generated expression; determine whether the implementation can preserve Text without unpacking and converting it through fromString. Done means the example works without an explicit unpack or custom instance and the resulting conversion is not needlessly round-tripped.

Written by the indexing model from the issue text.