Add help (:?)
Add sugar for defining a variable

‘x = <expr>’ is short for ‘:a { x = <expr>; }’.  Note that the
right-hand side refers to the original scope, so you get:

  nix-repl> x = 1
  nix-repl> x = x + 1
  nix-repl> x
  2

rather than an infinite recursion.
Temporary hack to parameterize nixpkgs