cache function names and file names

[sj-i]

• function names and file names can be retrieved from zend_function_entry
• they can be cached by using the address of zend_function_entry as a key at least in CLI
• in mod_php or fpm, the cache must be invalidated per request

[sj-i]

sapi_globals is a public symbol that remains in the stripped binary. Its members include global_request_time, which is filled with a timestamp in the request initialization sequence in SAPI. We can use it to detect request boundaries. Then, we can retrieve the function name, class name, and file name from a cache by simply looking at the address value of zend_execute_data.func. It is also probably possible to have a reasonably high hit rate cache for opline as well.

This is another way to reduce the number of calls to process_vm_readv() significantly #158

This resolves #63 also.

[sj-i]

Note that global_request_time is microseconds from the UNIX epoch in both mod_php and fpm.