Make hvcat for StaticArray and UniformScaling inputs produce a StaticArray

[sethaxen]

hcat and vcat seem to be specialized for StaticArray blocks, but hvcat does not seem to be specialized. Examples

julia> using StaticArrays

julia> A, B = @SMatrix(randn(2, 2)), @SMatrix(randn(2, 2));

julia> [A B] # fine
2×4 SMatrix{2, 4, Float64, 8} with indices SOneTo(2)×SOneTo(4):
  1.26449    0.426149  -1.15767    0.218855
 -0.816403  -0.241287  -0.663465  -0.367645

julia> [A; B] # fine
4×2 SMatrix{4, 2, Float64, 8} with indices SOneTo(4)×SOneTo(2):
  1.26449    0.426149
 -0.816403  -0.241287
 -1.15767    0.218855
 -0.663465  -0.367645

julia> [A B; A B] # Matrix
4×4 Matrix{Float64}:
  1.26449    0.426149  -1.15767    0.218855
 -0.816403  -0.241287  -0.663465  -0.367645
  1.26449    0.426149  -1.15767    0.218855
 -0.816403  -0.241287  -0.663465  -0.367645

julia> [A B; I] # Matrix
6×4 Matrix{Float64}:
  1.26449    0.426149  -1.15767    0.218855
 -0.816403  -0.241287  -0.663465  -0.367645
  1.0        0.0        0.0        0.0
  0.0        1.0        0.0        0.0
  0.0        0.0        1.0        0.0
  0.0        0.0        0.0        1.0

[mateuszbaran]

#811 was an attempt do do something like that. It seems like constant propagation should be enough in simple cases. I guess for up to four matrices const prop should work? I'm not sure whether doing such a partial solution is even desirable.