Lightweight PHP extension to hard-cap function call depth.

This extension will throw an Error if the function call stack reaches a depth of 256 calls. This is currently non-configurable, but usable for most cases.

• recursionguard.enabled (default 1): Controls whether the extension will guard for recursions even when loaded (useful for static builds).

Recursion errors can be difficult to debug because they cause PHP to crash without any stack trace when something overrides zend_execute_ex. Therefore, it's desirable to have this barf at a hard limit instead of segfaulting without apparent reason.

However, the only way to do this before now was the xdebug extension, which causes huge performance degradation and as such is unsuitable for production. This extension keeps to the point, causing a hit of only 10% extra function call overhead, versus xdebug's 2000+% overhead. This additional overhead becomes insignificant outside of synthetic benchmarks.