Optimize var_export

[kamil-tekiela]

The first commit replaces sprintf with memset.

The second commit replaces php_addcslashes and php_str_to_str with a custom function based on the smart_str_append_escaped function from zend_smart_str.c. At one of the 3 call sites, the php_str_to_str was absent, and I didn't know what to do, which is why I added the flag. This makes the most sense to me, but I could also make it separate from the new function.

I don't know whether buffer_append_spaces should have inline or not. I assume it shouldn't.

In my simple round of testing, this proved to make var_export 3x faster.