Using is_ascii_string to check encoding

[S-H-GAMELINKS]

Some code has this code to check encoding is ascii.

rb_enc_str_coderange(result) != ENC_CODERANGE_7BIT

But, already implemented is_ascii_string function that check ascii encoding in internal/string.h.

static inline bool
is_ascii_string(VALUE str)
{
    return rb_enc_str_coderange(str) == ENC_CODERANGE_7BIT;
}

I thought better to these check code replace and using is_ascii_string function.

And, #5554 add build dependence but that was unneeded.
So, I recreate pull request.