`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.
Inline structures have no instance variables, no finalizer, and offer as much space as possible in RBASIC object. This means 24 bytes on 64-bit platforms and 12 bytes on 32-bit platforms.
mruby-inline-struct gem is only provided for testing.