To override a non-builtin with a function, use command. For example:

ls() { command ls -l; }

which is the equivalent of alias ls='ls -l'.

command works with builtins as well. So, your cd could also be written as:

 cd() { echo before; command cd "$1"; echo after; }

To bypass a function or an alias and run the original command or builtin, you can put a \ at the beginning:

\ls # bypasses the function and executes /bin/ls directly