Bugfixes
- fix compilation with GCC7 (include necessary headers)
- fix build with recent V8 5.9 versions (libv8_libplatform not found)
- adapt exception_start_column.phpt test to changed output of recent V8 5.9 versions
- fix compilation with PHP 7.1.3
Refactorings
- drop TSRM stuff unneeded since PHP 7
- remove unused variables
- make memory_limit a size_t internally (and throw if negative values are provided on PHP interface)