Problem:
vim.iter has both `rfind()` and various `*back()` methods, which work
in "reverse" or "backwards" order. It's inconsistent to have both kinds
of names, and "back" is fairly uncommon (rust) compared to python
(rfind, rstrip, rsplit, …).
Solution:
- Remove `nthback()` and let `nth()` take a negative index.
- Because `rnth()` looks pretty obscure, and because it's intuitive
for a function named `nth()` to take negative indexes.
- Rename `xxback()` methods to `rxx()`.
- This informally groups the "list-iterator" functions under a common
`r` prefix, which helps discoverability.
- Rename `peekback()` to `pop()`, in duality with the existing `peek`.
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