Make iocamljs tutorial notebook

[djs55]

To make a 'tutorial' notebook in the style of [avsm/ocaml-dockerfile] we need an implementation of the external functions:

From [mirage/io-page#23]:

• caml_alloc_pages

From [mirage/ocaml-cstruct#63]:

• caml_blit_bigstring_to_bigstring
• caml_blit_bigstring_to_string
• caml_blit_string_to_bigstring
• caml_compare_bigstring
• caml_fill_bigstring

From vchan #71 :

• stub_atomic_fetch_and_uint8
• stub_atomic_or_fetch_uint8

From [ocsigen/js_of_ocaml#314]:

• caml_ba_uint8_{get,set}{16,32,64}

These are still undefined but I don't seem to need them (yet):

• caml_int32_bswap
• caml_int64_bswap
• caml_bswap16

Outstanding issues:

• use ppx rather than camlp4 (as suggested by @avsm)
• sane build runes: are they possible?
• something about security (as suggested by @talex5) -- this might need to be boosted into a top-level issue since the security story is currently weak

[djs55]

Prototype notebook is here:

http://djs55.github.io/ocaml-vchan/#

[hhugo]

caml_int32_bswap, caml_int64_bswap, caml_bswap16 are now provided by js_of_ocaml

[djs55]

Great, thanks!