Add plugin command support for nix repl

- Refactor the repl core into libexpr
- Use dependency injection via std::function to provide the completion
  functions from editline so we don't introduce extra dependencies for
  libexpr
- Add a RegisterReplCmd analogous to RegisterPrimOp for repl commands
- Refactor: get rid of the "ugly" global curRepl and replace it with a
  trick with closures on the nix side (that are effectively globals, but
  unique per repl user and thus not as ugly ;p)
- Rip out readline support since there appears to be no build system
  support for it and it is thus dead code
- Integration test this new plugin functionality
- Document it

doc/manual/command-ref/conf-file.xml

@@ -611,8 +611,8 @@ password <replaceable>my-password</replaceable>
         plugins may construct static instances of RegisterPrimOp to
         add new primops or constants to the expression language,
         RegisterStoreImplementation to add new store implementations,
-        RegisterCommand to add new subcommands to the
-        <literal>nix</literal> command, and RegisterSetting to add new
+        RegisterReplCmd to add new commands to the
+        <literal>nix</literal> REPL, and RegisterSetting to add new
         nix config settings. See the constructors for those types for
         more details.
       </para>