opt_each is badly named and an actual opt_each should be provided

[jbigot]

What opt_each currently handles is a situation where we can have each of these two options:

key: "a single value"

or

key:
 - "a first value"
 - "a second value"
 - "and some more..."

This would be better named one_or_each.

On the other hand, we miss a function to handle the case where we look for 0 or more values, that is, where the key might be completely missing.
This could maybe be written like that:

// for a list
each(tree, or_none, [](PC_tree_t value) {
}
// for a mapping
each(tree, or_none, [](PC_tree_t key, PC_tree_t value) {
}

that way, the variant could be easily added to all functions.

[benoitmartin88]

update deisa plugin when this issue is fixed.