Add ASAN support to the zend allocator

[danog]

This pull request adds ASAN support to the zend allocator, by automatically poisoning all unused pages, chunks and heap management structures before exiting the alloc, free, etc (all ZEND_API) functions.

Internally, the implementation uses the following rules:

• Always poison memory (re)allocated by private (non-ZEND_API) allocation functions before returning it
• Unpoison memory (re)allocated by public (ZEND_API) allocation functions before returning within the ZEND_API function
• Always poison freed memory
• Always poison unused memory during reallocation (where new_size < old_size)
• When accessing private heap structures and fields, always unpoison before accessing and repoison immediately after
  • An exception to the above (for simplicity) is the main heap datastructure, which is poisoned only when entering a ZEND_API function and repoisoned before exiting

This is what allowed me to find #18833, before I found the fast shutdown workaround.