devel

Skip list priority implementation (#4287)

* add skip list implementation

* update taskvine

* format

* re-establish cursor creation

* save current cursor position for deletion

* add iteration macro

* use iterate macro

* fixes from review

* format

* malloc to xxalloc

* length to size

* fix memleak

* fix mem-error, use after possible free

* remove left-over debug messages

* empty ready task on delete