… buffer size

The interned string buffer is organized as a header + a hash table + a
zend_string arena. Hash slots point to the arena, but are represented as 32bit
offsets from the buffer, which limits the maximum buffer size to about 4GiB.
However zend_strings are 8-byte aligned in the buffer, so we can compress the
3 lower bits. This allows to increase the maximum supported interned string
buffer size from 4095 MiB to 16367 MiB.