Refactor out map.el usage

After some profiling, it turns out map-put and map-delete are 5-7x
slower (more on Emacs 25) than delq, setf/alist-get and add-to-list for
small lists (under 250 items), which is exactly how I've been using
them.

The only caveat is alist-get's signature is different on Emacs 25, thus
a polyfill is necessary in core-lib.

modules/lang/csharp/config.el

@@ -1,6 +1,7 @@
 ;;; lang/csharp/config.el -*- lexical-binding: t; -*-
 
-(map-put auto-mode-alist '"\\.shader$" 'dshader-mode) ; unity shaders
+;; unity shaders
+(add-to-list 'auto-mode-alist '("\\.shader$" . shader-mode))
 
 
 (def-package! omnisharp