. Generators implementation was improved using the new VM stack. Now it's a bit more clear and faster.

EG(arg_types_stack) is now also backed up when generators are used. This
allows the use of yield in nested method calls.

This commit adds two new functions to the zend_ptr_stack API:

    zend_ptr_stack_push_from_memory
    zend_ptr_stack_pop_into_memory

both taking the following arguments:

    zend_ptr_stack *stack, int count, void **pointers

This fixes bug #60097.

Before two global variables CG(heredoc) and CG(heredoc_len) were used to
track the current heredoc label. In order to support nested heredoc
strings the *previous* heredoc label was assigned as the token value of
T_START_HEREDOC and the language_parser.y assigned that to CG(heredoc).

This created a dependency of the lexer on the parser. Thus the
token_get_all() function, which accesses the lexer directly without
also running the parser, was not able to tokenize nested heredoc strings
(and leaked memory). Same applies for the source-code highlighting
functions.

The new approach is to maintain a heredoc_label_stack in the lexer, which
contains all active heredoc labels.

As it is no longer required, T_START_HEREDOC and T_END_HEREDOC now don't
carry a token value anymore.

In order to make the work with zend_ptr_stack in this context more
convenient I added a new function zend_ptr_stack_top(), which retrieves the
top element of the stack (similar to zend_stack_top()).