Generators are now automatically detected by the presence of a `yield`
expression in their body.
This removes the ZEND_SUSPEND_AND_RETURN_GENERATOR opcode. Instead
additional checks for ZEND_ACC_GENERATOR are added to the fcall_common
helper and zend_call_function.
This also adds a new function zend_generator_create_zval, which handles
the actual creation of the generator zval from an op array.
I feel like I should deglobalize the zend_create_execute_data_from_op_array
code a bit. It currently changes EG(current_execute_data) and
EG(opline_ptr) which is somewhat confusing (given the name).
