Add mrb_get_args_a()

[sasq64]

This pull request adds a new function, mrb_get_args_a(), which works exactly like mrb_get_args() except that it takes
the pointer arguments as an array instead of varargs.

I needed to add this for my mruby binder library that I want to release publicly. It allows you to write code like:

(from my test cases)

    auto* ruby = mrb_open();

    auto* person_class = mrb::make_class<Person>(ruby, "Person");
    mrb::add_method<Person>(
        ruby, "age=", [](Person* p, int age) { p->age = age; });
    mrb::add_method<Person>(
        ruby, "age", [](Person const* p) { return p->age; });

    mrb::add_method<Person>(ruby, "copy_from", [](Person* p, Person* src) {
        p->name = src->name;
        p->age = src->age;
    });

    mrb::add_method<Person>(
        ruby, "dup", [](Person const* p) { return new Person(*p); });

    auto other_age = mrb_load_string(ruby,
        "person = Person.new ; person.age = 5 ; other = "
        "Person.new ; other.copy_from(person) ; person.age = 2 ; other.age");
    CHECK(mrb::value_to<int>(other_age) == 5);

[sasq64]

Good points. Updated my branch.

[sasq64]

@dearblue Anything else I need to do ?

[dearblue]

I'm a third party just like you, so I don't have the authority to commit.
Please wait for @matz's decision.

[lopopolo]

having an MRB_API (public API) with a va_list parameter will make it difficult to provide an API and ABI compatible MRB API implementation outside of C. Is it intentional that this function is marked as part of the public API?

[matz]

Functions with va_args should be specific to C API (by nature). You can safely ignore it.
And is mrb_get_args_a() above fit your need to provide Rust API?

[lopopolo]

And is mrb_get_args_a() above fit your need to provide Rust API?

yes it does. thank you!

[matz]

OK, I will merge this PR (with some updates).