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/lexer.scm

@@ -252,7 +252,15 @@
 ;;; Main lexer routine, which is given a port and does look for the next
 ;;; token.
 
+(define lexical-binding-regexp
+  (make-regexp
+   "-\\*-(|.*;)[ \t]*lexical-binding:[ \t]*([^;]*[^ \t;]).*-\\*-"))
+
 (define (lex port)
+  (define (lexical-binding-value string)
+    (and=> (regexp-exec lexical-binding-regexp string)
+           (lambda (match)
+             (not (member (match:substring match 2) '("nil" "()"))))))
   (let ((return (let ((file (if (file-port? port)
                                 (port-filename port)
                                 #f))
@@ -283,11 +291,19 @@
       (case c
         ;; A line comment, skip until end-of-line is found.
         ((#\;)
-         (let iterate ()
-           (let ((cur (read-char port)))
-             (if (or (eof-object? cur) (char=? cur #\newline))
-                 (lex port)
-                 (iterate)))))
+         (if (= (port-line port) 0)
+             (let iterate ((chars '()))
+               (let ((cur (read-char port)))
+                 (if (or (eof-object? cur) (char=? cur #\newline))
+                     (let ((string (list->string (reverse chars))))
+                       (return 'set-lexical-binding-mode!
+                               (lexical-binding-value string)))
+                     (iterate (cons cur chars)))))
+             (let iterate ()
+               (let ((cur (read-char port)))
+                 (if (or (eof-object? cur) (char=? cur #\newline))
+                     (lex port)
+                     (iterate))))))
         ;; A character literal.
         ((#\?)
          (return 'character (get-character port #f)))