`Fixed` does not fully support `fraction bits >= storage bits - 1`

Somehow related to #311 as it also concerns the lack of generalization, but in the other direction. If precision is really important, but range is not, we want to use all the bits to represent small numbers accepting that larger numbers, like 1, can no longer be represented.

Let `b` be the number of bits of the storage type and `f` the number of bits of the fraction. For `f < b - 1` everything is fine:
```julia
using FixedPointNumbers, Unitful

julia> Fixed{Int8, 6}(1)
1.0Q1f6

julia> Fixed{Int8, 6}(1)u"m"
1.0Q1f6 m
```

However, for `f >= b - 1` things start to break down when using units:
```julia
julia> Fixed{Int8, 7}(1)
"0.992188"

julia> Fixed{Int8, 7}(1)u"m"
ERROR: MethodError: no method matching *(::String, ::Unitful.FreeUnits{(m,), 𝐋, nothing})
The function `*` exists, but no method is defined for this combination of argument types.

Closest candidates are:
*(::Any, ::Any, ::Any, ::Any...)
@ Base operators.jl:642
*(::Missing, ::Unitful.Units)
@ Unitful ~/.julia/packages/Unitful/8Qjvg/src/units.jl:94
*(::LinRange, ::Unitful.Units)
@ Unitful ~/.julia/packages/Unitful/8Qjvg/src/range.jl:143
...

Stacktrace:
[1] top-level scope
@ REPL[86]:1
```

The reason seems to be the additional quotes which are printed for `f = 7` instead of `f = 6`, as can be seen above or more clearly below
```julia
julia> sprint(show, Fixed{Int8, 6}(1))
"1.0Q1f6"

julia> sprint(show, Fixed{Int8, 7}(1))
"\"0.992188\""
```