Start updating for the latest Rust builds

[emk]

This looks like a really cool library, and I'd love to try it! Thank you for all the work you've put in, and I appreciate how hard it's been to keep things building over the last couple of months.

I haven't managed to get this building with the latest Rust nightlies yet (I'm blocked by the Rust compiler bug https://github.com/rust-lang/rust/issues/25279), but I've made a fair bit of progress. This pull request contains all the build fixes I've figured out so far, and I hope to add more in the future if I make further progress.

Once again, many thanks!

[emk]

OK, at this point, all of the crates build correctly, and all the unit tests pass. However, both of the example programs still fail with errors like the following:

/home/.../jit.rs/examples/hello_world.rs:22:19: 22:34 error: the trait `jit::compile::Compile<'_>` is not implemented for the type `fn(&str)` [E0277]
/home/.../rust-libs/jit.rs/examples/hello_world.rs:22         let sig = get::<fn(&str)>();

For some reason, the following declarations don't seem to be creating the necessary Compile instances:

compile_func!(fn() -> R, fn() -> R, extern fn() -> R);
compile_func!(fn(A) -> R, fn(A) -> R, extern fn(A) -> R);
compile_func!(fn(A, B) -> R, fn(A, B) -> R, extern fn(A, B) -> R);
compile_func!(fn(A, B, C) -> R, fn(A, B, C) -> R, extern fn(A, B, C) -> R);
compile_func!(fn(A, B, C, D) -> R, fn(A, B, C, D) -> R, extern fn(A, B, C, D) -> R);

I'll poke at this a bit later.

[TomBebbington]

Thanks for your contributions! I've been busy recently because of exams but thanks a lot for the necessary updates, I'll see what else needs to be done to get it to compile

[emk]

Thank you for merging my work, and for your library! I'm looking forward to trying out some rather weird ideas with libjit in Rust once it's working again.