more builtin array method

[am-cid]

no descriptions means the same as python's behavior

• [x] prepend() prepends items to start of the array
  • uses insert(0) under the hood
• [x] pop() removes the last element in the list ALWAYS and does not return anything
• [x] index() works the same but instead of raising when cannot find the value, it will return -1
• [x] extend()
• [x] prextend() like extend but at the beginning of the list
• [x] clear()
• [x] count()