Allow 2-arg splice! on arbitrary index iterables (#34524)

We have deleteat!(vec::Vector, itr), but it returns the modified vector.
If you instead want to obtain the delted elements, there is
splice!(::Vector, ::UnitRange) as a special case of the three-argument
version that inserts additional elements where the old ones were
deleted. However, these is no two argument splice! for arbitrary
iterables, so probably the best way to write that currently is:

```
inds = collect(itr)
vals = A[inds]
deleteat!(A, inds)
vals
```

which is both less efficient and more verbose than ideal. I'm wondering
if perhaps deleteat! should have been returning the deleted elements,
but that's not a change we can make in 1.x. Instead, I'm proposing
here to extend the 2 argument (but not the 3 argument for obvious reasons)
variant of splice! to arbitrary iterables.

base/array.jl

@@ -1273,12 +1273,16 @@ Stacktrace:
 deleteat!(a::Vector, inds) = _deleteat!(a, inds)
 deleteat!(a::Vector, inds::AbstractVector) = _deleteat!(a, to_indices(a, (inds,))[1])
 
-function _deleteat!(a::Vector, inds)
+struct Nowhere; end
+push!(::Nowhere, _) = nothing
+
+function _deleteat!(a::Vector, inds, dltd=Nowhere())
     n = length(a)
     y = iterate(inds)
     y === nothing && return a
     n == 0 && throw(BoundsError(a, inds))
     (p, s) = y
+    p <= n && push!(dltd, @inbounds a[p])
     q = p+1
     while true
         y = iterate(inds, s)
@@ -1295,6 +1299,7 @@ function _deleteat!(a::Vector, inds)
             @inbounds a[p] = a[q]
             p += 1; q += 1
         end
+        push!(dltd, @inbounds a[i])
         q = i+1
     end
     while q <= n
@@ -1444,6 +1449,8 @@ function splice!(a::Vector, r::UnitRange{<:Integer}, ins=_default_splice)
     return v
 end
 
+splice!(a::Vector, inds) = (dltds = eltype(a)[]; _deleteat!(a, inds, dltds); dltds)
+
 function empty!(a::Vector)
     _deleteend!(a, length(a))
     return a

test/arrayops.jl

@@ -1342,6 +1342,7 @@ end
             @test a == [acopy[1:(first(idx)-1)]; repl; acopy[(last(idx)+1):end]]
         end
     end
+    @test splice!([4,3,2,1], [2, 4]) == [3, 1]
 end
 
 @testset "filter!" begin