[P2] No way to find the index of an element in a vector

[ecto]

index-of works on strings (returning the byte index or -1), but
there is no equivalent for vectors. find returns the matching
element rather than its position; indices is not in the VM
builtin set.

Proposal

• [index-of xs elem] -> Int (the position, -1 if absent), or
  return Option Int, mirroring the chosen Option story.
• Or add position / find-index.

Acceptance criteria

• Builtin available in the VM and documented in ref/builtins.loon.
• Tests for hit, miss, first element, last element, duplicates.