Coverts a function into an action that can be rendered into HTML. A
unique, hard to guess string is generated for the function, and a
function is added to the session hashtable under this string. The
string is then returned. When later requests come in,
'get-request-action' machinery determines if the action string that
came with the request is stored in the hashtable, and if so, invokes
the stored function.

'action-fn' - A function that will be called if the user initiates
appropriate control (link, form, etc.) GET and POST parameters will be
passed to this function as keyword arguments by the framework.

'action-code' - The code to use for an action (if not specified
make-action generates a unique value for each action). Note, if you
don't provide a hard to guess code ('generate-action-code' is used by
default), the user will be vulnerable to an attack where a malicious
attacker can attempt to guess a dangerour action id and send the user
a link to it. Only use guessable action codes for GET actions.