Implement mrb_protect.

[take-cheeze]

This pull request implements mrb_protect, mrb_ensure, mrb_rescue, mrb_rescue_exceptions.
(mrb_rescue_exceptions is implementation of rb_rescue2.)
The 4 APIs are placed to "mruby-error" mrbgem which is included to 'default' gembox for now.

The changes to Build#compile_as_cxx is for missing directory when generating C++ file in some cases.

[mattn]

@take-cheeze ++

[cremno]

Great! But I see one issue. Your test code even contains it:

return mrb_rescue_exceptions(mrb, protect_cb, b, protect_cb, r, E_TYPE_ERROR, NULL);

NULL can expand to 0 (int), so one always has to write at least (struct RClass*)0. Why not change the function to take a pointer to an array and its length instead? Then it'd be type-safe and mruby can't and doesn't have to be compatible to CRuby anyway. However there's also a new downside: an extra variable or, if a C99 compiler is used, a compound literal is needed . There's also the mrb_funcall() compromise (length + varargs).

[jbreeden]

Hurray! Regarding @cremno's comment, +1 for the array argument idea. It could still be NULL terminated, without sacrificing type safety (or go the length + array route... either way).

Thanks, @take-cheeze!

[take-cheeze]

@cremno
Array of exception classes seems nice.
The variadic API is confusing and I've changed the API name too.
I'm thinking of following API:

MRB_API mrb_value mrb_rescue_exceptions(
    mrb_state *mrb, mrb_func_t body, mrb_value b_data,
    mrb_func_t rescue, mrb_value r_data,
    mrb_int classes_len, struct RClasss **classes);

[take-cheeze]

Done changing to array of classes API. ( 15c869f )

[Asmod4n]

yay

[matz]

merged