The "a"/"*" specifier of the `mrb_get_args()` function will now return `const mrb_value *`.
This is because it is difficult for the caller to check if it is an array object and write-barrier if necessary.
And it requires calling `mrb_ary_modify()` on the unmodified array object, which is also difficult (this is similar to #5087).
`mrb_get_arg1()` raises `ArgumentError` if the method does not receive one
argument.
And replaces all `mrb_get_args(mrb, "o", &arg)` by the new function.
The binary sizes (gems are only `mruby-bin-mruby`) are reduced slightly in
my environment than before the introduction of new specifiers/modifiers
(5116789a) with this change.
------------+-------------------+-------------------+--------
BINARY | BEFORE (5116789a) | AFTER (This PR) | RATIO
------------+-------------------+-------------------+--------
mruby | 593416 bytes | 593208 bytes | -0.04%
libmruby.a | 769048 bytes | 767264 bytes | -0.23%
------------+-------------------+-------------------+--------
BTW, I accidentally changed `tasks/toolchains/visualcpp.rake` at #4613,
so I put it back.
dearblue