Changing/switching the name of the $$is_a_module property #2210

I like @meh's idea to have $$is_namespace. Having is_a_module and is_module is confusing.

By the way, can we make it a runtime helper? Like Opal.truthy?.
We could compileOpal.namespace?(obj) to something like obj.$$is_class || obj.$$is_module because it seems that it's not necessary to have all 3 props. Of course, it requires us to write this JS manually in runtime.js (then maybe it's even better to make a JS method Opal.is_namespace(obj))

Yeah we could actually go with a bunch of helpers, since they're really short they're 100% going to get inlined either way.

@elia raised some good points against namespace, there seem to be a bunch of cases where is_module and is_class are checked with logic other than AND.

@elia @meh From what I see everywhere is:

1. $$is_module
2. $$is_class
3. $$is_module || $$is_class -> $$is_namespace
4. !$$is_module && !$$is_class -> !($$is_module || $$is_class) -> !$$is_namespace (inverted case Remove :BLOCK_GIVEN lexer token #3)

Looks like we can go with one runtime helper.