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.

You don't have to define explicit `host` build target any more.

- `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.

That describes the changes in `mruby3`.

If this assertion fails, you have to define `MRB_USE_METHOD_T_STRUCT`.

It's the first change of renaming configuration options to `MRB_XXX` to
`MRB_USE_XXX` or `MRB_NO_XXX`.

On platforms where `time_t` is unsigned, negative time can be a result
of integer casting. Out-of-range error is too strict for those cases.
This fix does not address wrong time value in `MRB_WORD_BOXING`. It will
be addressed later (by introducing "big" integers).

On some platforms, `sizeof(mrb_value) > sizeof(void*)*3`, which makes
`MRB_ARY_EMBED_LEN_MAX` zero. And zero sized array cause compile errors.

Remove `#ifdef` from `union mrb_value_`.