Replace cl with cl-lib for emacs 27

[takaxp]

Package cl is deprecated.

[technician77]

I'm not a programmer, so I might not even comment here, but if you just apply the change to the .el and recompile you get an error about a "first" function not defined. The error causes the compile to fail. If I revert back to cl it compiles without an error.

[half-shell]

This works for me on GNU Emacs 27.1.
It gets rid of cl package being deprecated, an issue mentioned here and here, with a comment suggesting packages be updated to use cl-lib instead of cl.

@takaxp Is this still alive? Can we merge this or is there another supported repo that I am not aware of?

[takaxp]

Package cl is deprecated is still shown even if GNU Emacs 27.1 is used :(

[stephan-cr]

As @technician77 correctly points out, there's more work to do. All cl functions/macros have the cl- prefix in cl-lib. first needs to be cl-first. The same applies for second, third and probably many more. Right now it seems to work, because something else is loading cl.

The following functions need a prefix:

• assert
• first
• second
• third
• pushnew

Furthermore, lexical-let needs to be replaced by something else.

[pataquets]

Looks like this PR is superceded by #22. If there's nothing else here, @takaxp: fancy to close this in favor of #22?