Problem: The eval.txt help file is way too big.
Solution: Move the builtin function details to a separate file.
1cae5a0a03
Note: Neovim-specific references to |functions| were changed to
|builtin-functions|. This included updates to:
1. test/functional/vimscript/functions_spec.lua
2. test/functional/vimscript/eval_spec.lua
3. runtime/doc/lua.txt
