Make mapreduce also work with Range{BigInt}

I noticed the following behavior: when calling one of

    prod(1:big(16))
    reduce(*, 1:big(16))

this results in

```
ERROR: `mapreduce_impl` has no method matching mapreduce_impl(::IdFun, ::MulFun, ::UnitRange{BigInt}, ::Int64, ::BigInt)
 in _mapreduce at reduce.jl:168
 in prod at reduce.jl:265
```

(On the other hand,

    prod(1:big(15))
    reduce(*, 1:big(15))
    foldl(*, 1:big(16))
    factorial(big(16))

all work fine.)

I propose this commit, which fixes that behavior, but should

    mapreduce_pairwise_impl(f, op, A::AbstractArray, ifirst::Int, ilast::Int, blksize::Int)
    mapreduce_seq_impl(f, op::AddFun, a::AbstractArray, ifirst::Int, ilast::Int)
    mapreduce_impl(f, op::AddFun, A::AbstractArray, ifirst::Int, ilast::Int)
    mapreduce_impl(f, op::MaxFun, A::AbstractArray, first::Int, last::Int)
    mapreduce_impl(f, op::MinFun, A::AbstractArray, first::Int, last::Int)
    mapreduce_impl(f, op::AndFun, A::AbstractArray, ifirst::Int, ilast::Int)
    mapreduce_impl(f, op::OrFun, A::AbstractArray, ifirst::Int, ilast::Int)

possibly also be changed in the same way, even though their last arguments' being Int does not break anything (that I could find)?