Consider changing "not-found" behavior of `N`/`.N`

[nick-paul]

Originally brought up by @BlazingTwist here

The N operator returns -1 if it cannot find a value in a list. More recently, negative index support was added to Aya. -1 is no longer a good candidate for a default value since it is a valid index.

Possible options:

• N return length(list)+1 for not found
  • More difficult to use N as a basic contains operator since the index will need to be compared with the length of the list instead of with a known constant
    • Current way to check if a value is in a list: N0<!
    • New way to check if a value is in a list (compare with length of list): N\.E@=
• Possibly utilize unused :N operator for plain contains checking (return 1 if item is in list, 0 otherwise)

[BlazingTwist]

• N return length(list)+1 for not found
  • More difficult to use N as a basic contains operator since the index will need to be compared with the length of the list instead of with a known constant

If you have the index-of operators return 0 list EB -, that index will be out of bounds while still supporting N0<!