C-h f and C-h w bindings in smex-prepare-ido-bindings problematic if user needs C-h to be backspace

[seabadger]

Some users rely on the C-h keybinding to act as backspace (e.g., if a keyboard does not have a backspace key, or the key is remapped at a low level to act like something else, as some people do with keyboards that use customizable firmware like QMK).

In such cases, the C-h... bindings set up by smex in smex-prepare-ido-bindings mean there's no way to erase backward at a smex prompt.
It would be helpful if these keybindings were somehow customizable/overridable/optional.