Mask certain keys

[kellerza]

This masks certain struct keys.

The use case was to print a yang.Entry from https://github.com/openconfig/goyang

Each yang.Entry / yang.Node etc can have a parent which seems to recursively cause repr.Print to go into a loop
This now works with

repr.Println(e, repr.MaskKey("Parent"))

Open to change the name MaskKeys / HideKeys etc.
Could also use ... as you currently use in the seen logic

[alecthomas]

repr is supposed to detect recursive loops, we should fix that rather than adding extra functionality to work around that

[alecthomas]

Is it possible to provide a minimal working example for a case where the recursion detection fails?

[alecthomas]

There's an existing test for this exact scenario of parent references, so there must be something else going on.