To silence some warnings. This change cancels part of 7ef3604134.

* `mrb_kwargs` structure reordered (`values` and `rest` come last)
* take symbols instead of C `char*`

In the past code, the current `callinfo (ci)` was modified, thus it was
possible to pop `ci` beyond the `cibase`, that could cause out of memory
bound access for the code like the following:

```ruby
def m2
  lambda {
    Proc.new {
      return :return # return from the method
    }
  }.call.call
  :never_reached
end

p m2
```

By definition `mrb_assert()` called only when `MRB_DEBUG` is defined too.
But make I wanted to make clear that the local variable `current_checkpoint_tag`
is only accessed when `MRB_DEBUG` is set by wrapping with `DEBUG_ONLY_EXPR()`.

To make packed inline symbols within 31 bits, because the new method
hash tables allows only 31 bits of symbols. They use top 1 bit to maek
unused slots.

Since `%u` of `mrb_sym` may be its MSB turned on.

`mrb_int()` includes implicit integer conversion, where `mrb_integer()`
does not. In this case, we know `obj` is an integer before hand.

Caused from combination of `mrb_int`, `int` and `size_t`..

Implements `catch`/`throw` non-local jump inherited from Lisp.

`catch([tag]) {|tag| block }  -> obj`

Example:
```
  catch(:foo) { 123 }               # => 123
  catch(:foo) { throw(:foo, 456) }  # => 456
  catch(:foo) { throw(:foo) }       # => nil
```

From human readable (ASCII) string representation to binary dump of
IEEE754 in little endian.

They are basically the copy of instance variable tables. On my Linux
box, memory consumption of `mrbtest` measured by `valgrind` is:

- old: 17,683,830 bytes
- new: 14,283,749 bytes

It uses `mrb_str_new_lit()` internally, but it doesn't need to express
it in the name of the function (macro).

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()`

So that you can omit `host` target. Now `host-debug` works.

Based on cremno/mruby@6bd0119

They used to return `mrb_value` but now return `mrb_sym` for consistency
with other `intern` functions. If symbols are not defined, `check`
functions return `0`, instead of `nil` in the past.

It causes API incompatibility but I believe few people use those
functions out of the core, and those changes are very easy to handle,
hopefully.

We call `mrb_intern_str()` later anyway, so there's no need to avoid
defining a new symbol here.

Based on cremno/mruby@d446192

They are not used from the beginning.

As described in ISO 15.2.30.

We still have `#define MRB_TT_FIXNUM MRB_TT_INTEGER` for compatibility.