native code syntax extensions

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

I've finally added support for loading all the syntax extensions as native 
code, instead of byte code (although that option remains in the myocamlbuild in 
case of disaster).

Since we're up to 10 extensions now (Bitstring, Lwt, Ulex, JSON, CSS, HTML, 
XML, Dyntype, Type_conv, and eventually the ORM), this has hugely decreased 
build time: by about 3x-4x for the main tree, and more for statically linked in 
modules.

But since it's so close to the tutorial, would be great if everyone could test 
their trees by pulling from mine and making sure the dynamic code loading works 
on their platform (I've tested on MacOS x64 and Linux x64). You just need to 
pull, make && make install, and perhaps build mirage-www.

Anil