[perl #130020] [RFC][@LARRY] Create a set of conventions to minimize impact internal changes to user's code

Spotted another case where there's impact: whether or not a routine is a multi can have large impact on user's code. For example here: https://stackoverflow.com/questions/48819031/where-did-my-perl-6-operator-go-after-i-defined-a-more-specific-multi/48827522

As part of conventions, we'd need to also figure out what we'll declare as multies and what as onlies.