Switch to caller-rooted arguments calling convention for imported OCaml functions

[tizoc]

Changes

Changes to simplify Rust->OCaml calls through the use of a caller-save calling convention, and handling unexpected exceptions with a panic!.

• Make ocaml! produce functions that expect rooted arguments.
• Make ocaml! functions return results directly, instead of wrapped in Result. panic! on unexpected exceptions.
• Implement a way to catch OCaml raised exceptions. (Nothing will be done here for now, users will have to resort to std::panic::catch_unwind).
• Remove ocaml_call! and ocaml_alloc! macros.
• Update docs and examples
• Make FromOCaml::from_ocaml take a ref instead of a copy.

[g2p]

FWIW, you don't have to pass self by reference or change as_* to to_* for OCaml types that are basically one usize, Clippy gave that advice due to a recently introduced bug: rust-lang/rust-clippy#6316 (comment)

[tizoc]

Thanks, that is good to know, I'm pretty sure I saw that warning at some point. The as_ -> to_ changes are made were for consistency (the as_ ones I have return borrowed values, while the ones that changed from as_ -> to_ return new ones).