Provide a JNI dynamic-library interface to `enso_parser`.
# Important Notes
- The library can be built with: `cargo build -p enso-parser-jni`.
- A new `org.enso.syntax2.Parser` API is implemented on top of the JNI interface provided by `enso-parser-jni`.
- We are using the `jni` crate, since apparently Java cannot just call C-ABI functions. The crate is not well-maintained. I came across an obviously-unsound `safe` function, and found it was reported over a year ago, with a PR to fix: jni-rs/jni-rs#303. However our needs are simple. We can't trust any safety guarantees they imply, but I think we are unlikely to encounter any logic bugs using the basic bindings.
implement simple variable assignments and function definitions.
This implements:
- https://www.pivotaltracker.com/story/show/182497122
- https://www.pivotaltracker.com/story/show/182497144 (the code blocks are not created yet, but the function declaration is recognized.)
# Important Notes
- Introduced S-expression-based tests, and pretty-printing-roundtrip testing.
- Started writing tests for TypeDef based on the examples in the issue. None of them parse successfully.
- Fixed Number tokenizing.
- Moved most contents of parser's `main.rs` to `lib.rs` (fixes a warning).
