widemul: document and export it?

[StefanKarpinski]

The widemul function is pretty useful and we use it a fair bit in base. Should we commit to exporting and supporting it? I can't really think of any good reasons not to.

[JeffBezanson]

Yes, let's. Maybe there should also be a widen function that gives you a value in a reasonably-larger type.

[JeffBezanson]

And there is a decision on whether widen should be implemented as

widen(x) = convert(morebits(typeof(x)), x)

(morebits was added at some point)

[StefanKarpinski]

I think we should rename morebits to widen and then have this:

widen(::Type{Int8}) = Int
widen(::Type{Int16}) = Int
widen(::Type{Int32}) = Int64
widen(::Type{Int64}) = Int128
widen(::Type{Int128}) = BigInt
widen(::Type{BigInt}) = BigInt

widen(::Type{Float16}) = Float32
widen(::Type{Float32}) = Float64
widen(::Type{Float64}) = BigFloat
widen(::Type{BigFloat}) = BigFloat

widen{T<:Number}(x::T) = convert(widen(T), x)