Rename hook commands

[spl]

The current names preexec and precmd leave something to be desired. I suppose they come from zsh, but I'm not trying to stay true to zsh as bash-preexec did.

[spl]

My current thinking:

• preexec → before_command
• precmd → before_prompt

[spl]

Prefix the functions with the ”namespace“ prompt_hooks:: and rename the library “private” prefix from __bp to prompt_hooks::__.