Fix MSVC C++ ABI build. Close #2588.

Fix default gem loading in `generate_gem_table`.

get rid of shadowing variables (mrbgems)

fix strict aliasing rule violation

Mostly renaming, except that the definition of struct accessor methods
is now done in a new function.

get rid of shadowing variables

Mostly by renaming the shadowing variable.
If a shadowing variable was deleted,
the shadowed one can be used instead.

save and restore arena index to prevent arena overflow.

Use mrb_str_cat_lit() instead of mrb_str_cat_cstr() for string-literals.

Refactor kernel.c

delete obsolete macros (mrb_basic + mrb_object)

correctly validate symbol length

Over a year ago is not "soon" anymore. See:
88483fad
24366930

The comment didn't say why the length is reserved.
Using RITE_LV_NULL_MARK makes things a bit clearer.

This also fixes an off-by-one in mrb_check_intern().