birth,rebirth: Add cond-expand, string->es

`cond-expand' allows moving forward with implementing features based on the
current bootstrapped state of the system---rebirth will be able to
recursively compile itself and introduce new features along the way.

`string->es' allows outputting raw ECMAScript, which gives us more control
over the code that is generated without having to hard-code it in the
compiler itself.

* build-aux/bootstrap/birth.scm
  (fnmap)[cond-expand]: Always yield the empty string (do nothing).
* build-aux/bootstrap/rebirth.scm
  (fnmap)[cond-expand]: Expand `string->es' only.
  (fnmap)[string->es]: Add macro.

build-aux/bootstrap/birth.scm

@@ -377,6 +377,10 @@
       (("js:console")
        (string-append "console.log(" (map sexp->es args) ")"))
 
+      ;; always expands into nothing; this is just to facilitate its use
+      ;; moving forward
+      (("cond-expand") "")
+
       ;; yes, there are more important things to do until we get to the
       ;; point where it's worth implementing proper tail calls
       (("js:while")

build-aux/bootstrap/rebirth.scm

@@ -398,6 +398,19 @@
       (("js:console")
        (string-append "console.log(" (map sexp->es args) ")"))
 
+      ;; very primitive cond-expand
+      (("cond-expand")
+       (let* ((clause  (car args))
+              (feature (token-value (car clause)))
+              (body    (cdr clause)))
+         (case feature
+          (("string->es") (body->es body #f))
+          (else ""))))
+
+      ;; output raw code into the compiled ECMAScript (what could go wrong?)
+      (("string->es")
+       (token-value (car args)))
+
       ;; yes, there are more important things to do until we get to the
       ;; point where it's worth implementing proper tail calls
       (("js:while")