Guard against StackoverflowErrors: prevent packages from extending FileIO.load or FileIO.save

[timholy]

Follows up on #141 (comment)

[timholy]

CC @simonster

[timholy]

I should point out that this is a dangerous PR that I won't merge unless there's substantial buy-in from other folks. It will break JLD (and maybe many other packages) until they stop extending load and save:

julia> save("myfile.jld", Dict("x"=>5))
Error encountered while saving "/tmp/myfile.jld".
Fatal error:
ERROR: JLD writer error: module should not extend FileIO.save
  Will try next writer.

Though through the magic of dispatch, it works if you just try a second time:

julia> save("myfile.jld", Dict("x"=>5))

[ssfrr]

I think that if the FileIO API is that you shouldn't extend load and save but instead define your own, it seems reasonable to enforce that (especially if not doing so causes stack overflows).

If the affected package authors get a heads up they should be able to fix things on their side without breaking anything, before this gets merged. Another option could be to make it a warning at first, so there's a sort of deprecation period.

It would probably be good to include a specific note in the Implementing Loaders/Savers section of the README that says that your package should define load and save methods (but not export them).

[timholy]

Thanks @ssfrr. See #164. If we decide to go ahead with this PR, before merging I propose that we send something out to discourse and give folks a couple of weeks to adjust.

For the record:

julia> Pkg.dependents("FileIO")
50-element Array{AbstractString,1}:
 "Luxor"                 
 "Kpax3"                 
 "Images"                
 "ELF"                   
 "OMETIFF"               
 "MFCC"                  
 "GLAbstraction"         
 "NRRD"                  
 "LibSndFile"            
 "RData"                 
 "AndorSIF"              
 "JLD"                   
 "LasIO"                 
 "GLBooks"               
 "PkgBenchmark"          
 "Weber"                 
 "Meshes"                
 "WAV"                   
 "StatFiles"             
 "GLVisualize"           
 "GLWindow"              
 "RigidBodyTreeInspector"
 "TestImages"            
 "MeshIO"                
 "BedgraphFiles"         
 "Ogg"                   
 "VideoIO"               
 "TensorFlow"            
 "ProfileView"           
 "FeatherFiles"          
 "MP3"                   
 "JLD2"                  
 "CSVFiles"              
 "ColorSchemes"          
 "Opus"                  
 "SwiftObjectStores"     
 "ReferenceTests"        
 "QuartzImageIO"         
 "MetaImageFormat"       
 "FLAC"                  
 "ImageView"             
 "ImageMagick"           
 "ExcelFiles"            
 "ImagineFormat"         
 "Netpbm"                
 "Augmentor"             
 "Caesar"                
 "GslibIO"               
 "EMIRT"                 
 "RDatasets"             

We could download all of these and see which ones seem to be extending FileIO's methods. This won't catch private repos, but would be better than nothing. I feel guilty that we let this go so long without documenting it unambiguously.

[rapus95]

Stumbled over this, looked into the changes, found the ancient 2017 syntax. Happy to have newer better syntax.
Anyway, are there similar plans or was it just forgotton to be closed?

[timholy]

#295 implemented a warning