Convenience initializers for map and set

[Iainmon]

It would be nice to have many constructors for map, set, and list that accept a variety of equivalent representations of the data structure. Such as

var ages = [
    ("Iain",21),
    ("Kassia",20),
    ("Nick",23),
    ("Saul", 82),
    ("Martin", 55)
];

proc map.init(entries: [?d] ?t) {
    type kt = entries.first[0].type;
    type vt = entries.first[1].type;
    this.init(kt,vt);
    for (k,v) in entries {
        this[k] = v;
    }
}

var ageMap = new map(ages);

and overloaded operators like

ageMap += ("Cray", 51);
ageMap += [(k,v) in ageMap.items()] ("_" + k,v);

list has some convenient initializers, and it would be nice to extend that to map and set.

[lydia-duncan]

We actually removed some operator definitions that took another map as an argument, over confusion about what should happen when keys existed in both maps involved in the operation.