bin_opt_info: print binary append optimisation

[jonatanklosko]

The Constructing Binaries describe how the runtime system and the compiler optimise binary appends to avoid copying.

It would be useful for bin_opt_info to hint when the optimisation is applied. It would also be useful to print when it is not applied, but that may be too verbose.

Context

Recently we changed a function from private to exported and that prevented the optimisation from being applied. Comparing byte-code, I could spot the relevant difference, but having bin_opt_info hint about it would help to diagnose such cases. See dashbitco/lazy_html#19.

[bjorng]

Thanks for the suggestion. We have added it to our backlog.