Klist improvements

[tecywiz121]

Adds some new operations for klists:

• kl_length: Returns the number of elements in the list
• kl_unshiftp: Prepends to the beginning of the list
• kl_delete_after: Removes the node after the given one
• kl_insertp_after: Inserts an item after the given one

Fixes a couple bugs:

• Failed malloc and family functions are handled properly
• Use-after-free in kl_destroy

[tecywiz121]

Would love it if someone from this project took a look.