Remove inner constructor for ExactReal

[dpsanders]

This was preventing

ExactReal{Int64}(3)

which should be allowed.

[OlivierHnt]

I think this was done on purpose to avoid silent errors, e.g.

julia> ExactReal{Float64}(1//3)
ExactReal{Float64}(0.3333333333333333)

[Kolaru]

I think David's example should indeed be allowed, and Olivier's forbidden.

To make it so, we can add a second inner constructor:

struct ExactReal{T<:Real} <: Real
    value :: T

    ExactReal{T}(value::T) where {T<:Real} = new{T}(value)
    ExactReal(value::T) where {T<:Real} = new{T}(value)
end

[dpsanders]

Thanks @Kolaru, I added that inner constructor instead.

[Kolaru]

Quickly added simple tests, and LGTM, thanks :)