Consistent index edge-case behaviour

[pbiggar]

When there's a function that takes a list/array/string and an index, and the index is outside the bounds of the list, some functions will raise, some wrap, some do the thing at the start/end based on where the index is, and some do the same thing regardless of whether its out-of-bounds.

They should all behave the same on all platforms. What should that behaviour be?

My preference would be, in order:

1. normalize to valid bound
2. normalize with wrapping
3. raise an error if out of bounds
4. use option/result
5. silently return the input for out of bounds

By normalize-to-valid-bound I mean:

let index = 
  if index < 0
  then 0
  else min (length, index)

By normalize-with-wrapping I mean:

let index = 
  if index < 0
  then max (length + index), 0
  else min (length, index)

[pbiggar]

Also, if the same function exists for Strings, lists and/or arrays, they should behave the same.