♻️ Extract `rpgdm-core` and address deprecations

This PR has two commits

♻️ Favor cl- functions

When requiring rpgdm.el and it's "child" packages, I encounter the following warnings:

Warning: ‘destructuring-bind’ is an obsolete alias (as of 27.1); use ‘cl-destructuring-bind’ instead.

Warning: ‘incf’ is an obsolete alias (as of 27.1); use ‘cl-incf’ instead.

Warning: ‘decf’ is an obsolete alias (as of 27.1); use ‘cl-decf’ instead.

Warning: ‘defstruct’ is an obsolete alias (as of 27.1); use ‘cl-defstruct’ instead.

Since we've already required the cl package, this should be a noop change.  ## ♻️ Extract `rpgdm-core.el'

Prior to this commit, I experienced the following use case:

Given I have the following code:

(require 'rpgdm-dice "~/git/emacs-rpgdm/rpgdm-dice.el")
(require 'rpgdm-tables "~/git/emacs-rpgdm/rpgdm-tables.el")
(require 'rpgdm-tables-dice "~/git/emacs-rpgdm/rpgdm-tables-dice.el")
(require 'rpgdm-tables-freq "~/git/emacs-rpgdm/rpgdm-tables-freq.el")
    
(setq rpgdm-base "~/git/emacs-rpgdm/")

And I call rpgdm-tables-load And load all the tables.

When I then call rpgdm-tables-choose

Then I get the following error:

let*: Symbol’s function definition is void: rpgdm-message

With this commit I'm able to skip requiring rpgdm and thus only use a segment of the rpgdm package ecosystem.