Add a slow path in ultra2ultra transforms?

[jishnub]

The following works on v0.14:

julia> FastTransforms.ultra2ultra(ones(4), 3, 1)
4-element Vector{Float64}:
  4.0
 11.0
  6.0
 10.0

but not on v0.15:

julia> FastTransforms.ultra2ultra(ones(4), 3, 1)
ERROR: AssertionError: abs(λ₁ - λ₂) < 1
Stacktrace:
 [1] plan_th_ultra2ultra!(#unused#::Type{Float64}, ::Tuple{Int64}, λ₁::Int64, λ₂::Int64)
   @ FastTransforms ~/.julia/packages/FastTransforms/jlbLV/src/toeplitzhankel.jl:229
 [2] th_ultra2ultra(::Vector{Float64}, ::Int64, ::Int64)
   @ FastTransforms ~/.julia/packages/FastTransforms/jlbLV/src/toeplitzhankel.jl:286
 [3] ultra2ultra(::Vector{Float64}, ::Int64, ::Vararg{Int64}; z::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ FastTransforms ~/.julia/packages/FastTransforms/jlbLV/src/FastTransforms.jl:129
 [4] ultra2ultra(::Vector{Float64}, ::Int64, ::Vararg{Int64})
   @ FastTransforms ~/.julia/packages/FastTransforms/jlbLV/src/FastTransforms.jl:129
 [5] top-level scope
   @ REPL[2]:1

Perhaps there could be a slower path for the general cases, and a faster path for the Toeplitz-Hankel case?