Small Elm interpreter made in Rust

I lost interest on this project, because its main purpose was to be used as an scripting engine in Amethyst, but Amethyst will take months before it's ready to support scripting in a useful way.

So if anyone has a reason to use this project for anything else just open an issue.

Right now the project is close to completion, but will has some things that need work, for example:

• Fix several TODOs in the code for edge cases
• Replace serde_json with other serde plugin that uses a more compact binary format
• Fix closures not correctly collecting values fom they environment
• Benchmarking and improving the performance of the library
• Add support for FFI
• Add let expressions in the interpreter
• Add test with large code examples, the project has a lot of tests but they only cover lexing, parsing and type resolution, ignoring execution.

• Better and more consistent error reporting for different types of errors
• Add more info when a runtime error occurs
• Benchmarking every step in the interpreter
• Add Array, Process, Task and Platform modules from elm core
• More functions for Rust interop, like casting rust values to elm values and vice versa.

• Lexing/Tokenizing work pretty well, except for weird unicode characters.
• Parsing seems to work ok and it complete, but I haven't tested with large codebases, just elm core.
• Static analysis and type inference work most of the time, if a function doesn't have a type definition it can only use other functions that have a type definition or that are defined before.
• Execution has issues with closures, and let expressions are not implemented, this is the less tested part of the project.
• Rust interop allow to register rust function with Runtime.register_fn(function_ptr) but only if they use simple types as arguments and return
• Rust interop also allow to register function of type fn(&mut Interpreter, &[Value]) -> Result<Value, ElmError> allowing to interact directly with elm values