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/compile-tree-il.scm

@@ -50,7 +50,8 @@
             compile-defmacro
             compile-defun
             #{compile-`}#
-            compile-quote))
+            compile-quote
+            compile-%set-lexical-binding-mode))
 
 ;;; Certain common parameters (like the bindings data structure or
 ;;; compiler options) are not always passed around but accessed using
@@ -66,6 +67,8 @@
 
 (define always-lexical (make-fluid))
 
+(define lexical-binding (make-fluid))
+
 ;;; Find the source properties of some parsed expression if there are
 ;;; any associated with it.
 
@@ -245,7 +248,10 @@
            (let ((always (fluid-ref always-lexical)))
              (or (eq? always 'all)
                  (memq sym always)
-                 (get-lexical-binding (fluid-ref bindings-data) sym))))))
+                 (get-lexical-binding (fluid-ref bindings-data) sym)
+                 (and
+                  (fluid-ref lexical-binding)
+                  (not (global? (fluid-ref bindings-data) sym module))))))))
 
 (define (split-let-bindings bindings module)
   (let iterate ((tail bindings)
@@ -854,6 +860,12 @@
     ((,val)
      (make-const loc val))))
 
+(defspecial %set-lexical-binding-mode (loc args)
+  (pmatch args
+    ((,val)
+     (fluid-set! lexical-binding val)
+     (make-void loc))))
+
 ;;; Compile a compound expression to Tree-IL.
 
 (define (compile-pair loc expr)