Layout: replace API used to couple classes with layouts

Eliminate the need to provide the destination object prototype when the
layout is defined.  Change the API so destination objects are created by
a function instead of cloning a prototype.  Rename the function that
associates layouts with constructors to be more descriptive of what it
does.

README.md

@@ -157,17 +157,17 @@ representing the union and the variants:
     function Vu32(v) { this.u32 = v; }
     util.inherits(Vu32, Union);
     lo.bindConstructorLayout(Vu32,
-                             Union._layout.addVariant('w'.charCodeAt(0), lo.u32(), 'u32'));
+                             Union.layout_.addVariant('w'.charCodeAt(0), lo.u32(), 'u32'));
 
     function Vs16(v) { this.s16 = v; }
     util.inherits(Vs16, Union);
     lo.bindConstructorLayout(Vs16,
-                             Union._layout.addVariant('h'.charCodeAt(0), lo.seq(lo.s16(), 2), 's16'));
+                             Union.layout_.addVariant('h'.charCodeAt(0), lo.seq(lo.s16(), 2), 's16'));
 
     function Vf32(v) { this.f32 = v; }
     util.inherits(Vf32, Union);
     lo.bindConstructorLayout(Vf32,
-                             Union._layout.addVariant('f'.charCodeAt(0), lo.f32(), 'f32'));
+                             Union.layout_.addVariant('f'.charCodeAt(0), lo.f32(), 'f32'));
 
     var v = Union.decode(Buffer('7778563412', 'hex'));
     assert(v instanceof Vu32);
@@ -179,7 +179,7 @@ representing the union and the variants:
     assert.equal(v.t, 0xa5);
     assert.deepEqual(v.u8, [0xa5, 0xa5, 0xa5, 0xa5]);
 
-    var b = new Buffer(Union._layout.span);
+    var b = new Buffer(Union.layout_.span);
     v = new Vf32(23.625);
     v.encode(b);
     assert.equal(Buffer('660000bd41', 'hex').compare(b), 0);