MEDIUM: pattern: turn the pattern chaining to single-linked list

MEDIUM: pattern: turn the pattern chaining to single-linked list

It does not require heavy deletion from the expr anymore, so we can now
turn this to a single-linked list since most of the time we want to delete
all instances of a given pattern from the head. By doing so we save 32 bytes
of memory per pattern. The pat_unlink_from_head() function was adjusted
accordingly.