Switch from smex to amx

[bcc32]

Spacemacs's smex and ivy layers configure package smex for persisting
M-x history among other things. However, smex has not been updated in
a long time (10 years) and amx is an actively developed fork of it.

Amx supports at least migrating the smex-save-file automatically to
amx-save-file, but function calls would need to be updated.