prebirth: Add newlines between body expressions

Just makes it easier to look at for debugging.

* build-aux/bootstrap/prebirth.js (Compiler#_bodyToEs): Add newlines.

build-aux/bootstrap/prebirth.js

@@ -455,11 +455,11 @@ class Compiler
         // the result (that is, an array of compiled s-expressions) is
         // joined semicolon-delimited, with a `return' statement preceding
         // the final expression
-        return js.reduce( ( result, s, i ) =>
+        return js.map( ( s, i ) =>
         {
             const ret = ( i === ( js.length - 1 ) ) ? "return " : "";
-            return result + "    " + ret + s + ";";
-        }, "" );
+            return `    ${ret}${s};`;
+        } ).join( '\n' );
     }