Simplify dropdims(::Transpose) and insertdims(::PermutedDimsArray) etc.

[mcabbott]

Before, these wrap the wrapper in a ReshapedArray:

julia> dropdims([1,2,3]'; dims=1)
3-element reshape(adjoint(::Vector{Int64}), 3) with eltype Int64:
 1
 2
 3

julia> insertdims(PermutedDimsArray([1 2; 3 4], (2,1)); dims=2)
2×1×2 reshape(PermutedDimsArray(::Matrix{Int64}, (2, 1)), 2, 1, 2) with eltype Int64:
[:, :, 1] =
 1
 2

[:, :, 2] =
 3
 4

After:

julia> dropdims([1,2,3]'; dims=1)
3-element Vector{Int64}:
 1
 2
 3

julia> insertdims(PermutedDimsArray([1 2; 3 4], (2,1)); dims=2)
2×1×2 PermutedDimsArray(::Array{Int64, 3}, (2, 3, 1)) with eltype Int64:
[:, :, 1] =
 1
 2

[:, :, 2] =
 3
 4

This is not always faster, alone, but the hope is that the simpler return type will usually be faster downstream. (Especially with e.g. CuArrays, where wrapping twice causes them to miss specialised methods.)

julia> P13 = PermutedDimsArray(randn(2,1,3,1,4), (4,5,2,1,3));

julia> @btime dropdims($P13; dims=(1,3));
  189.628 ns (10 allocations: 192 bytes)  # before, ReshapedArray(PermutedDimsArray(Array{Float64, 5}))
  272.895 ns (10 allocations: 256 bytes)  # after, PermutedDimsArray(Array{Float64, 3})

julia> A13 = collect(P13);

julia> @btime dropdims($A13; dims=(1,3));
  172.333 ns (11 allocations: 240 bytes)

It appears that the compiler can predict the type when dims is constant. More stringent tests (or ways to do better) would be welcome:

julia> drop1(x) = dropdims(x; dims=(1,));

julia> @code_warntype drop1(P13)  # before
MethodInstance for drop1(::PermutedDimsArray{Float64, 5, (4, 5, 2, 1, 3), (4, 3, 5, 1, 2), Array{Float64, 5}})
  from drop1(x) @ Main REPL[12]:1
Arguments
  #self#::Core.Const(drop1)
  x::PermutedDimsArray{Float64, 5, (4, 5, 2, 1, 3), (4, 3, 5, 1, 2), Array{Float64, 5}}
Body::Base.ReshapedArray{Float64, 4, PermutedDimsArray{Float64, 5, (4, 5, 2, 1, 3), (4, 3, 5, 1, 2), Array{Float64, 5}}, NTuple{4, Base.MultiplicativeInverses.SignedMultiplicativeInverse{Int64}}}
1 ─ %1 = (:dims,)::Core.Const((:dims,))
...

julia> @code_warntype drop1(P13)  # after
MethodInstance for drop1(::PermutedDimsArray{Float64, 5, (4, 5, 2, 1, 3), (4, 3, 5, 1, 2), Array{Float64, 5}})
  from drop1(x) @ Main REPL[240]:1
Arguments
  #self#::Core.Const(drop1)
  x::PermutedDimsArray{Float64, 5, (4, 5, 2, 1, 3), (4, 3, 5, 1, 2), Array{Float64, 5}}
Body::PermutedDimsArray{Float64, 4, (4, 2, 1, 3), (3, 2, 4, 1), Array{Float64, 4}}
1 ─ %1 = (:dims,)::Core.Const((:dims,))
...

[mcabbott]

Xref #45793 which added insertdims. And I requested review from a few of those who commented on that PR.