Deprecate cat(dim, Xs...) to cat(Xs..., dims=dim)

[mbauman]

Fixes #27100. I have also started on a bigger refactor here that will more easily allow for custom arrays to extend concatenations, but that ended up being not as simple as I had hoped, so this PR simply moves to the dims keyword argument and hides all internal dispatch so Base only exposes one cat method without restrictions on its arguments for now.

[mbauman]

Man, rough go on CI: FreeBSD timed out, both Travis Linuxes got stuck/segfaulted waiting for a git username/password, and CircleCI isn't even loading for me, so I cannot see what was going on in the 32bit build there.

[KristofferC]

32bit CI loads for me and got:

 Got exception ErrorException("Process timed out possibly waiting for a response. Process output found:\n\"\"\"\n\r\nsignal (15): Terminated\r\nin expression starting at no file:0\r\n_ZN4llvm13ConstantRange21makeAllowedICmpRegionENS_7CmpInst9PredicateERKS0_ at /tmp/julia/bin/../lib/julia/libLLVM-6.0.so (unknown line)\r\n_ZN4llvm13ConstantRange24makeSatisfyingICmpRegionENS_7CmpInst9PredicateERKS0_ at /tmp/julia/bin/../lib/julia/libLLVM-6.0.so (unknown line)\r\n_ZN4llvm13ConstantRange19makeExactICmpRegionENS_7CmpInst9PredicateERKNS_5APIntE at /tmp/julia/bin/../lib/julia/libLLVM-6.0.so (unknown line)\r\n\n\"\"\"") outside of a @test
  Process timed out possibly waiting for a response. Process output found:
  """
  

  signal (15): Terminated

  in expression starting at no file:0

  _ZN4llvm13ConstantRange21makeAllowedICmpRegionENS_7CmpInst9PredicateERKS0_ at /tmp/julia/bin/../lib/julia/libLLVM-6.0.so (unknown line)

  _ZN4llvm13ConstantRange24makeSatisfyingICmpRegionENS_7CmpInst9PredicateERKS0_ at /tmp/julia/bin/../lib/julia/libLLVM-6.0.so (unknown line)

  _ZN4llvm13ConstantRange19makeExactICmpRegionENS_7CmpInst9PredicateERKNS_5APIntE at /tmp/julia/bin/../lib/julia/libLLVM-6.0.so (unknown line)

  
  """
  Stacktrace:
   [1] error(::String, ::String) at ./error.jl:42
   [2] (::getfield(Main.Test36Main_LibGit2_libgit2.LibGit2Tests, Symbol("##5#8")){Int32,Cmd,Array{Any,1},getfield(Main.Test36Main_LibGit2_libgit2.LibGit2Tests, Symbol("#format_output#7")){Bool},Base.GenericIOBuffer{Array{UInt8,1}}})(::RawFD, ::Base.TTY) at /tmp/julia/share/julia/stdlib/v0.7/LibGit2/test/libgit2.jl:93
   [3] with_fake_pty(::getfield(Main.Test36Main_LibGit2_libgit2.LibGit2Tests, Symbol("##5#8")){Int32,Cmd,Array{Any,1},getfield(Main.Test36Main_LibGit2_libgit2.LibGit2Tests, Symbol("#format_output#7")){Bool},Base.GenericIOBuffer{Array{UInt8,1}}}) at /tmp/julia/share/julia/test/TestHelpers.jl:37
...

[Keno]

I think we may have to increase those timeouts for libgit2

[JeffBezanson]

Note on the merge conflict: I moved this code to util.jl (with the rest of the text-color-related code, which I did to resolve a bootstrap order thing, but it makes sense anyway).

[iagobaapellaniz]

cat(k, A...)](@ref) ...

it should be cat(A...; dims = k), isn't it?