Avoid generating code that involves deprecated functions (in String mostly)

[nberth]

• String.{{un}capitalize,lowercase} -> String.{{un}capitalize,lowercase}_ascii
• String.create -> Bytes.create
• Also fix useless `with' clauses

Note the previous changes require OCaml ≥ 4.03 for compiling camlidl itself. Yet all this allows a smooth transition towards generating code that comply with immutability of strings in OCaml ≥ 4.06 (another pull request soon to come).

• Add a .gitignore