Add example with a naive Clojure->Rust transpiler

[thomasantony]

This is something I hashed out in a couple of hours. The idea is to somehow create a rust binary from a clojure source file that embeds the parsed clojure data structures and then "eval"s it. This may be a completely stupid idea as I do not know if this will translate well for all the clojure constructs.

The transpiler currently creates "working" rust code for examples/hello_world.clj. It generates valid Rust for examples/strings.clj but the program fails at runtime. To run the hello world example:

cargo run --example transpiler examples/hello_world.clj examples/codegen_hello_world
cargo run --example codegen_hello_world

I also updated a couple of macros to use $crate instead of crate in its module paths.

Maybe just embedding the source directly as a string inside the binary is a far better idea, but then that's not really transpiling, is it?