SRFI 111 boxes implementation is not portable

[leppie]

It explicitly imports (chezscheme) .

Here is a simple portable implementation: https://github.com/IronScheme/chez-srfi/blob/master/%253a111/boxes.sls

[arcfide]

I think it would best to retain the Chez Scheme specific implementation for Chez Scheme to preserve optimization potential and the like, but I'd accept a PR with a portable implementation. Good catch BTW.