chore(editor): Replace smex with amx package

Smex has not been updated since late 2015. Amx was originally a fork of
smex, but as since evolved much further into it's own thing.

Among other things is supports other completing backends, and ivy with
it's fuzzy matching feels rather nice. I'm keeping ido-mode for
everything else, but specifically for M-x I'll give ivy a try.

modules/navigation/siren-helm.el

@@ -24,7 +24,6 @@
   (helm-cleanup . siren-helm--show-treemacs-maybe)
 
   :bind
-  ("M-x" . helm-M-x)
   ("C-t" . helm-imenu)
   ("C-c t" . helm-imenu-anywhere)
   ("C-x C-f" . helm-find-files)