Properly bind prefix key commands in alchemist-iex-mode-map

[akirak]

Hello, I set alchemist-key-command-prefix to (kbd "C-,"), and it turns out that the implementation in the master branch may not properly bind some keys in alchemist-iex-mode-map. With the config described above, some of the keybindings look like [67108908]ir, which is awkward. This PR fixes the issue.