ref #9280
Introduce the `vim.compat` module, to help environments with system Lua
5.2+ run the build/tests. Include the module implicitly in all tests.

ref #8677
legacy `vim` module:
    beep
    buffer
    command
    dict
    eval
    firstline
    lastline
    line
    list
    open
    type
    window

Make the code run both on Lua 5.1 (which is the default for Neovim, and
is what LuaJIT provides) and Lua 5.2+.

    Error detected while processing function man#open_page[58]..<SNR>54_put_page:
    line    8:
    E5105: Error while calling lua chunk: /usr/share/nvim/runtime/lua/man.lua:165: Vim(let):E805: Using a Float as a Number