Add camlp5 8.03 support, update make switch to use it

[aqjune]

This patch adds camlp5 8.03 support and does the following updates:

• Update make switch to use it
• Move all pa_j_* files into a new directory pa_j, and add README that informally describes how these files are created.
• Update the label of Github CI from camlp5 8.02 to camlp5 8.03 .

Using camlp5 8.03 is necessary to enable using native OCaml REPL in the future.

[aqjune]

I checked that s2n bignum proofs checked successfully with this patch.

[jrh13]

Thank you, this is a very worthwhile reorganization and improvement.

I've checked that the new "make switch" combination works on several platforms, and that direct build still works equally well the the "native" OCaml and camlp5 versions on those platforms (ranging from OCaml 4.05 to 5.1).

[aqjune]

Thanks. :)