Add PHP 7.3 support

[rybakit]

Ref: https://blog.remirepo.net/post/2018/07/02/PHP-extensions-status-with-upcoming-PHP-7.3

[Totktonada]

A user observed the build fail:

/var/tmp/tarantool/src/tarantool.c: In function ‘zim_Tarantool___construct’:
/var/tmp/tarantool/src/tarantool.c:1153:20: error: lvalue required as left operand of assignment
   GC_REFCOUNT(&le) = 1;
                    ^
Makefile:193: recipe for target 'src/tarantool.lo' failed

It seems that we should use GC_SET_REFCOUNT(&le, 1); on PHP 7.3, see https://github.com/couchbase/php-couchbase/pull/22/files#diff-0681ab7d7064f2e8b779813c9c114005R288

[bigbes]

https://cismon.net/2018/02/06/Changes-of-Zend-API-in-PHP-7.3/

Update by @Totktonada: The link is died. Actual one is https://github.com/CismonX/cismon.net/blob/master/source/_posts/Changes-of-Zend-API-in-PHP-7.3.md

[SonSergei]

See #148

[Totktonada]

Fixed in php7-v2 branch in 0.3.0-24-gd1f2a0e. It'll be switched to master in the scope of #137.