Fix ambiguity problems in sum_structs

[Tortar]

So that this

using StaticArrays
using MixedStructTypes

const Point3 = SVector{3}
const Vector3 = SVector{3}

abstract type AbstractShape{T<:AbstractFloat} end

@sum_structs Shape{T<:AbstractFloat, SL, SR} <: AbstractShape{T} begin
	struct Box{T<:AbstractFloat}
	    fDimensions::Vector3{T}
	end
	struct Tube{T<:AbstractFloat}
	    rmin::T
	    rmax::T
	    z::T
	end
	struct Cone{T<:AbstractFloat}
	    r::T
	    z::T
	end
	struct BooleanUnion{T<:AbstractFloat, SL<:AbstractShape{T}, SR<:AbstractShape{T}}
	    left::SL 
	    right::SR
	end
end

box1 = Box{Float64}((1,2,3))

would work. Now we have:

ERROR: MethodError: Box{Float64}(::var"##Box#225"{Float64}) is ambiguous.

Candidates:
  Box{T}(fDimensions) where T<:AbstractFloat
    @ Main ~/.julia/dev/MixedStructTypes/src/SumStructs.jl:222
  Box{T}(ht::var"##Box#225"{T}) where T
    @ Main ~/.julia/dev/SumTypes/src/sum_type.jl:179

Possible fix, define
  Box{T}(::var"##Box#225"{T}) where T<:AbstractFloat

Stacktrace:
 [1] Box{Float64}(fDimensions::Tuple{Int64, Int64, Int64})
   @ Main ~/.julia/dev/MixedStructTypes/src/SumStructs.jl:224
 [2] top-level scope
   @ REPL[7]:1

...after MasonProtter/SumTypes.jl#78