Addressed an issue where existing programs linking `libmruby.a` could only
be built by adding `<build-dir>/include` to compiler's include path.

Where fixnum overflow can happen.

`sizeof(rand_state)` had been bigger than `sizeof(void*)*3`. Changed
random number generator to `Xorshift96` on 32 bit platforms.

- 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.

For efficiency with `MRB_WORD_BOXING` (implement type predicate macros for
all `enum mrb_vtype`).

Array index became potentially negative because `uint32_t` is cast to
`mrb_int`.

When the size of Xorshift128 seed (`sizeof(uint32)*4`) is bigger than
ISTRUCT_DATA_SIZE, `Random` uses Xorshift96 instead.

Now `rand` can be used with `MRB_WITHOUT_FLOAT`; ref #4576

The purpose is to clarify the error if there is a needed/conflicts
configuration at compile time.

The ISO standard does not include implicit type conversion using
`to_int`. This implicit conversion often causes vulnerability.
There will be no more attacks like #4120.

In addition, we have added internal convenience method `__to_int` which
does type check and conversion (from floats).

It reduces the memory consumption and sometimes improve the
performance as well.  For example, the consumed memory size
of `bench/bm_ao_render.rb` is reduced from 1.2GB to 1GB, and
its total execution time become 18.795 sec from 22.229 sec.

This reverts commit 5cdcce8d.

if you see compiler errors due to this commit, you'd better to use array-modifying
functions, e.g. mrb_ary_set() or mrb_ary_push(), otherwise you might see nasty
GC bugs in the future.  if you are sure what you are doing, replace `RARRAY_PTR(ary)`
by `mrb_ary_ptr(ary)->ptr`. but be warned.

GLOBAL_RAND_SEED_KEY is no longer used.