add a bit of widening to reductions, to make up for the new
type-preserving arithmetic.

allow reducedim(+, A, 2), which previously gave
```
ERROR: `reducedim_init` has no method matching reducedim_init(::IdFun, ::Function, ::Array{Uint8,2}, ::Int64)
 in reducedim at reducedim.jl:217
```