make cumsum call cumsum!

[stevengj]

See #9665: this makes cumsum! and cumsum use the same pairwise code for 1d arrays (whereas previously cumsum! used a completely different codepath based on the one for multidimensional arrays, which just does naive summation). Ideally, we'd use the pairwise algorithm for multidimensional arrays too, but that requires a much bigger code rewrite. Also updates the docs to mention using cumsum! to control the precision.

cc: @timholy, @ivarne

[timholy]

LGTM

[tkelman]

Wonder why appveyor is not building on this? Might need a force re-push to try again?

[tkelman]

Should this be backported along with #9650?

[stevengj]

It could be, I suppose. It's an accuracy improvement without changing any API.

[tkelman]

nevermind on backporting, caused a test failure:

exception on 9: ERROR: test error during isequal(cumsum(A),A1)
`cumsum` has no method matching cumsum(::Array{Int64,3})
 in anonymous at test.jl:62
 in do_test at test.jl:37
 in anonymous at no file:394
 in runtests at C:\cygwin64\home\MPC_CAR\julia\test\testdefs.jl:5
 in anonymous at multi.jl:855
 in run_work_thunk at multi.jl:621
 in anonymous at task.jl:855
while loading arrayops.jl, in expression starting on line 388
ERROR: test error during isequal(cumsum(A),A1)
`cumsum` has no method matching cumsum(::Array{Int64,3})
while loading arrayops.jl, in expression starting on line 388
while loading C:\cygwin64\home\MPC_CAR\julia\test\runtests.jl, in expression starting on line 39