`signed` and `unsigned` don't accept integer types from this package as arguments

[matthias314]

In Julia Base, signed and unsigned accept integer types as arguments. It would be good if BitIntegers.jl could support this.

julia> signed(UInt8)
Int8

julia> signed(UInt256)
ERROR: MethodError: no method matching rem(::Type{UInt256}, ::Type{Int256})
The function `rem` exists, but no method is defined for this combination of argument types.

[rfourquet]

Closed in #2, I will make release today.