added default keyword parameters to ICollector to help write correct and safe scope and stack pushes and pops. shorthands for manual enter/leave, push/pop pairs

[jurgenvinju]

This is not a priority. Added some enhancements to ICollector be able to write:

void collect(current: (Program) `begin <Declarations decls> <{Statement  ";"}* body> end`, Collector c){
    c.scope(current, () {
        collect(decls, body, c);
    });
}

instead of:

void collect(current: (Program) `begin <Declarations decls> <{Statement  ";"}* body> end`, Collector c){
    c.enterScope(current);
        collect(decls, body, c);
    c.leaveScope(current);
}

The scope shorthand (and its siblings lubScope, compositeScope, compositeLubScope, ...) also wraps a try-finally block around the block, such that in the presence of unexpected exceptions the scope stacks are properly unwound automatically.

The changes are located purely in the abstract definition of ICollector as a list of keyword parameters that call the other function fields. So the scope keyword parameter calls the enterScope and leaveScope positional field, etc. There is no impact on other code in typepal.