e.g. symbols like "foo[]=" make invalid C codes

The original PR was skipping Ruby comments as well, but caused some
issues in test suites.

- `mrb_check_intern()` to return `mrb_value`
- `mrb_intern_check()` to return `mrb_sym` [NEW]

Other new functions:

- `mrb_intern_check_cstr()`
- `mrb_intern_check_str()`

Rename new functions:

- `mrb_convert_type(mrb,val,type,tname,method)`
   => `mrb_type_convert(mrb,val,type,tname,method)`
- `mrb_check_convert_type(mrb,val,type,tname,method)`
   => `mrb_type_convert_check(mrb,val,type,tname,method)`

Old names are defined by macros (support `tname` drop and
`char*` => `mrb_sym` conversion).

On platforms where `sizeof(long)` is 4, casting `(long)` can lose data
or sign information.

- Implement `Rational()` in `C`.
- Use `float` to `rational` conversion function taken from:
  https://rosettacode.org/wiki/Convert_decimal_number_to_rational#C

The mixture causes warnings on 64 bit Windows (VC).

You can now use `NEXT` within `switch` statement like 7c087eb.

On non-`gcc` compatible environment, `NEXT` is translated to `break`.

Linux `atan2(3)` man page says:

```
If  y is positive infinity (negative infinity) and x is positive infinity,
+pi/4 (-pi/4) is re‐ turned.
```

But on Microsoft VC/MinGW, `atan2()` returns `NaN` if either of
arguments is infinite. So we skip those tests on the platforms.

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.