API to retrieve opcode operand and extended_value meaning

I've fixed few issues reported by Bob.

Hi,

Please review the PR (for master only).
It adds an API call to get information about opcode operand and
extended_value meaning. This API should be useful for phpdbg, xdebug and
others.
Once these extension switch to new API, they will support most new opcodes
out of the box.

https://github.com/php/php-src/pull/1610

Comments and additional ideas are welcome.

Thanks. Dmitry.