Enhanced Standard Library

What? A Lua library collecting useful utilities and functions for writing Lua in/for Neovim.

Why? The vim module already provides a decent standard library for this purpose, but there's still a lot of functionality that gets commonly reimplemented by plugin authors - things like functional programming combinators (map, fold, etc.), utilities for working with paths, wrappers around vim.loop.spawn for working with jobs, etc. To encourage code reuse, it would be useful to have a "standard library++" including these tools.

Potential existing implementations: https://github.com/nvim-lua/plenary.nvim is already working toward this!

Potential pitfalls: What should be included/excluded to keep the library useful without bloat?