Support signed/unsigned reals, and a compact representation

[cjdoris]

Two issues, but both require new type parameters to InfExtended, which could become:

InfExtended{T<:Real, C, S, F}
  flag :: F
  finitevalue :: T
  ...
end

Firstly, C for "compact" is a boolean. When false then F = InfFlag and flag represents whether the value is finite, positive infinity or negative infinity, as is the current implementation (or will be after #4). When true then F = Nothing and we use typemax(T) and typemin(T) to represent infinity in the finitevalue. This is "compact" because sizeof(InfExtended{T, true, ...}) == sizeof(T), at the expense of losing two finite values from T.

• What should the default C be? Perhaps true whenever T is a bits type, and false otherwise?
• Make this default an overloadable function, so packages can define it for their own types?
• How can we let the user choose? Perhaps InfExtended(T, :compact=>true)? Or just InfExtended(T, true)?

Secondly S for "signed" is a boolean. When true then both positive and negative infinity are allowed values. When false then only positive infinity is allowed. Hence S should be true whenever T is a signed type.

• Clearly the default for S is whether or not T is a signed type.
• Do we ever let the user choose? I'm thinking no, but we could have a function issigned(T) which controls its behaviour.