Launch a separate toplevel for each HTML file.

The toplevel is executed in a separate process which can be started
and stopped at will (for each HTML file that executes a toplevel
sentence ATM).  The advantages are that the camlp4 extensions loaded
for a page do not influence the other pages and that the main "build"
program can be compiled to native code.
Better indent function definition.
Properly highlight ;; in programs.
Force the linking of some modules in Code_top for Camlp4.
Rename Main into Ocamlorg to avoid a clash with compiler-libs.
Improve the "foreach" tutorial.

In particular, make pa_foreach3 compile and use it in the toploop.