add partial document updates

[tomijaga]

Resolves #13

Operations added

    public type UpdateFieldOperations = {
        #set : (Candid.Candid);
        #inc : (Int);
        #dec : (Int);
        #mul : (Int);
        #div : (Int);
    };

    public type UpdateOperations<Record> = {
        #doc : Record;
        #ops : [(Text, UpdateFieldOperations)];
    };