emacs-compatible lexical binding

* module/language/elisp/bindings.scm (global?): New function.
* module/language/elisp/compile-tree-il.scm (lexical-binding): New
  variable.
  (bind-lexically?): If lexical binding is enabled, bind lexically
  unless a special binding exists.
  (compile-%set-lexical-binding-mode): New function.
* module/language/elisp/lexer.scm (lexical-binding-regexp): New
  variable.
  (lex): Return a `set-lexical-binding-mode!' token if a comment is
  found while reading the first line.
* module/language/elisp/parser.scm (get-expression): Add support for
  `set-lexical-binding-mode!' tokens.
* module/language/elisp/runtime/function-slot.scm: Import and re-export
  the `%set-lexical-binding-mode' special form.
* test-suite/tests/elisp-compiler.test
  ("Let and Let*")["lambda args inside lexical-let"]: Update.

module/language/elisp/runtime/function-slot.scm

@@ -41,7 +41,9 @@
                  (compile-defun . defun)
                  (compile-defmacro . defmacro)
                  (#{compile-`}# . #{`}#)
-                 (compile-quote . quote)))
+                 (compile-quote . quote)
+                 (compile-%set-lexical-binding-mode
+                  . %set-lexical-binding-mode)))
   #:duplicates (last)
   ;; special operators
   #:re-export (progn
@@ -64,6 +66,7 @@
                defun
                defmacro
                #{`}#
-               quote)
+               quote
+               %set-lexical-binding-mode)
   ;; functions
   #:re-export (apply))