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.

Also remove some unnecessary code.

https://bugs.ruby-lang.org/issues/9982

Trailing `.0` is removed from `Float#to_s` and `Float#inspect` at
9d08025b. However, I think the more human-readable format is better
for `Float#inspect`.

For example, in the `Float#to_s` format, the failure message is not
well understood when testing values including types by `eql?` (e.g.
`Numeric#step` test).

  ```ruby
  assert "example" do
    exp = 1.0
    act = 1
    assert_operator(exp, :eql?, act)  #=> Expected 1 to be eql? 1.
  end
  ```

* The `Fixnum` constant is now an alias for the `Integer` class.
* Remove `struct mrb_state::fixnum_class` member.
  If necessary, use `struct mrb_state::integer_class` instead.

  ```console
  $ bin/mruby -e 'p({1 => 2}.key?(1.0))'
  true
  ```

  ```console
  $ bin/mruby -e 'p({1 => 2}.key?(1.0))'
  false
  ```

close #3123

TODO: Unlike CRuby, mruby's `Proc#parameters` does not distinguish
required keyword arguments and optional keyword arguments currently.