prebirth: Allow nested defines

This just reorganizes things slightly to allow for nested `define's.  This
is a common means of encapsulation, and will Just Work™ because JS has
pretty much identical block scoping rules in this regard.

* build-aux/bootstrap/prebirth.js (compile): Invoke `#_sexpsToEs' directly
    instead of `#_cdfn' on tree.
  (_cdfn): Remove non-`define' check to proxy to `#_sexpsToEs'.
  (_sexpsToEs): Treat `define' as a special case, invoking `#_cdfn'.

build-aux/bootstrap/prebirth.js

@@ -328,7 +328,7 @@ class Compiler
     {
         // map every definition to a ES function definition and delimit them
         // (for readability) by two newlines
-        return tree.map( this._cdfn.bind( this ) )
+        return tree.map( this._sexpToEs.bind( this ) )
             .join( "\n\n" ) + "\n";
     }
 
@@ -351,12 +351,6 @@ class Compiler
             );
         }
 
-        // if it's not a definition, then it's a top-level application
-        if ( t[ 0 ].value !== 'define' )
-        {
-            return this._sexpToEs( t ) + ';';
-        }
-
         // e.g. (define (foo ...) body)
         const [ , [ { value: name }, ...params ], ...body ] = t;
 
@@ -496,6 +490,10 @@ class Compiler
             }
         }
 
+        if ( t[ 0 ].value === 'define' ) {
+            return this._cdfn( t );
+        }
+
         // simple function application (fn ...args)
         const [ { value: fn }, ...args ] = t;