#1911 - Type instability in genmat_fallback

[schillic]

Closes #1911.

# master
julia> @btime genmat(rand(Interval))
  423.462 ns (7 allocations: 464 bytes)

julia> @btime genmat(rand(BallInf, dim=100))
  48.197 μs (315 allocations: 98.44 KiB)

# this branch
julia> @btime genmat(rand(Interval))
  175.353 ns (4 allocations: 272 bytes)

julia> @btime genmat(rand(BallInf, dim=100));
  21.260 μs (7 allocations: 80.02 KiB)

julia> @code_warntype genmat(rand(Interval))
Variables
  #self#::Core.Compiler.Const(LazySets.genmat, false)
  H::Interval{Float64,IntervalArithmetic.Interval{Float64}}

Body::Array{Float64,2}
1 ─ %1 = LazySets.genmat_fallback(H)::Array{Float64,2}
└──      return %1

[mforets]

Nice!

[mforets]

LGTM (again)