extgen: Add support for init/minit/mshutdown

[withinboredom]

This PR adjusts extgen to handle:

• the case where the go code already contains an init() function.
• support for //export_php:module init to handle resource creation during module startup
• support for //export_php:module shutdown to handle resource cleanup during module shutdown

[withinboredom]

My last commit breaks classes, but is needed for functions-only extensions. I'll need to make that import conditional on whether or not there are classes present and add a test for it.

[withinboredom]

This one is probably ready for review now @dunglas

[alexandre-daubois]

This is really nice and certainly useful. Thanks!

Bikeshedding: should we call the directives //export_php:minit and mshutdown to match PHP's macros? Your proposition also suits me fine. It's just to see if someone would prefer this 🙂