prebirth: Add Prebirth facade

This provides the complete compiler output, which is consistent with the
output of Birth.  This will be useful for browser-based bootstrapping.

* build-aux/bootstrap/prebirth.js: Extract code into `Prebirth' class and
    use the class.
  (Prebirth): Add class.

build-aux/bootstrap/prebirth.js

@@ -655,33 +655,59 @@ const fnmap = {
 };
 
 
+/**
+ * Facade to make compilation as easy as `Prebirth#compile'
+ */
+class Prebirth
+{
+    /**
+     * Parse and compile SRC.
+     *
+     * The output will be wrapped in a self-executing function to restrict
+     * scope, and will be prefixed with LIB (which should be libprebirth.js).
+     *
+     * @param {string} src Prebirth Lisp source to compile
+     * @param {string} lib libprebirth.js
+     *
+     * @return {string} compiler output with trailing newline
+     */
+    compile( src, lib )
+    {
+        const p = new Parser();
+        const c = new Compiler( fnmap );
+
+        const tree = p.parseLisp( src );
+
+        // output libprebirth and compiled output, wrapped in a self-executing
+        // function to limit scope
+        return "(function(){" +
+            lib + '\n\n' +
+            c.compile( tree ) +
+            "})();\n";
+    }
+}
+
+
 
 /*
  * Prebirth was originally intended to be run via the command line using
  * Node.js.  But it doesn't have to be.  If you want, feel free to run it in
- * your web browser; you'll just have to instantiate your own objects.
+ * your web browser; you'll just have to instantiate your own Prebirth.
  */
 ( function ()
 {
-    if ( typeof process === 'undefined' )
+    if ( typeof process === 'undefined' ) {
-    {
+       return;
-        return;
     }
 
-    const p = new Parser();
+    const fs  = require( 'fs' );
-    const c = new Compiler( fnmap );
+    const src = fs.readFileSync( '/dev/stdin' ).toString();
+    const lib = fs.readFileSync( __dirname + '/libprebirth.js' ).toString();
 
-    const fs   = require( 'fs' );
+    const prebirth = new Prebirth();
-    const src  = fs.readFileSync( '/dev/stdin' ).toString();
-    const tree = p.parseLisp( src );
-    const lib  = fs.readFileSync( __dirname + '/libprebirth.js' ).toString();
 
-    // output libprebirth and compiled output, wrapped in a self-executing
+    // compile and output to stdout
-    // function to limit scope
+    process.stdout.write( prebirth.compile( src, lib ) );
-    process.stdout.write( "(function(){" );
-    process.stdout.write( lib + '\n\n' );
-    process.stdout.write( c.compile( tree ) );
-    process.stdout.write( "})();\n" );
 } )();