Choosing type of quantity value based on type used in construction

Arguably, when we do `1.0f0u"m"`, the floating point backing type should remain `Float32` rather than being converted to `Float64`:
```julia
julia> (1.0f0u"m").value
1.0
```

In general, there's a question of what type we should choose for the quantity value. In theory, we could even keep integers as integers, i.e. `1u"m"` would remain `1 m`. But perhaps we would want to enforce floating point, in which case something like `float(T)` might make sense.