- Integrate `Fixnum` and `Integer`
- Remove `Integral`
- `int / int -> int`
- Replace `mrb_fixnum()` to `mrb_int()`
- Replace `mrb_fixnum_value()` to `mrb_int_value()`.
- Use `mrb_integer_p()` instead of `mrb_fixnum_p()`

- `MRB_WITHOUT_FLOAT` => `MRB_NO_FLOAT`
- `MRB_USE_FLOAT` => `MRB_USE_FLOAT32`

The former is to use `USE_XXX` naming convention. The latter is to make
sure `float` is 32bit float and not floating point number in general.

`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.

`gcc -O3` raises error on truncation using `snprintf`.

`gcc -O3` raises error on truncation using `snprintf`.

`mrb_raisef()` only takes `%S` specifier. If you don't have extra
arguments, use `mrb_raise()`.

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.