The optimizations that vim.iter uses for array-like tables don't require
that the source table has no holes. The only thing that needs to change
is the determination if a table is "list-like": rather than requiring
consecutive, integer keys, we can simply test for (positive) integer
keys only, and remove any holes in the original array when we make a
copy for the iterator.
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`.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
If an iterator pipeline stage returns nil as its first return value, the
other return values are ignored and it is treated as if that stage
returned only nil (the semantics of returning nil are different between
different stages). This is consistent with how for loops work in Lua
more generally, where the for loop breaks when the first return value
from the function iterator is nil (see :h for-in for details).
