This reverts commit fd10c72319.
I thought it was OK to restrict index value within 1 byte, but in some
cases index value could be 16 bits (2 bytes). I had several ideas to
address the issue, but reverting `fd10c72` is the easiest way. The
biggest reason is `mruby/c` still supports `OP_EXT[123]`, so that they
don't need any additional work.
Dr Markus Kuhn published in 1999 an article [1] explaining in details
why we shouldn't use the ASCII grave accent (0x60) as a left quotation.
Backticks have been used most notably to produce nice-looking LaTeX
documents but it doesn't seem to be an issue on modern platforms and
for the oldest ones, there are workarounds as mentioned by Dr Kuhn.
[1]: https://www.cl.cam.ac.uk/~mgk25/ucs/quotes.html
dearblue