Fix build
Handle SIGINT to cancel the current line
Use readline history file
Don't exit on SIGINT during evaluation

However, this may leave thunks in black-holed state, so it's not
really safe.
Add a command :b to build a derivation
Add a command :s to start a nix-shell for a derivation
Load files specified on the command line

For example:

  $ nix-repl '<nixpkgs>' '<nixos>'
  Welcome to Nix version 1.6pre3215_2c1ecf8. Type :? for help.

  Loading ‘<nixpkgs>’...
  Added 3337 variables.

  Loading ‘<nixos>’...
  Added 7 variables.

  nix-repl>
Add basic variable name completion
Restore affinity