rebirth: Add `begin'

This isn't used yet, but it's a nice thing to have available.

* build-aux/bootstrap/rebirth/es.scm (begin): Add syntatic form.

build-aux/bootstrap/rebirth/es.scm

@@ -192,6 +192,13 @@
                 (map cdr clauses)))
      "}})()")
 
+
+   ;; We can just re-use `let' for `begin' since it already does exactly
+   ;; what we need it to
+   (define-macro (begin . exprs)
+     (`quote (let () (unquote@ exprs))))
+
+
    ;; We unfortunately have to worry about environment mutability in the
    ;; current implementation.  Since variables within environments are
    ;; implemented using ECMAScript's prototype chain, any sets affect the