Make the interpreter stdout/stderr/stdin agnostic

[CohenArthur]

In order to reach more platforms such as WASM or micro controllers, it would be nice to move away from the current implementation which relies on {e}println and the standard library's file reading abstraction. Instead, we should focus some of our efforts on adding two traits (with a default implementation) which would allow the interpreter to run on other platforms provided they implement that trait.

The ideal API for "libjinko" users (such as the interpreter) would be something like that:

- let mut ctx = Context::new();
+ let mut ctx = Context::<DefaultReader, WasmWriter>::new();

• Add a Reader/Input trait
  • Add a method to read a file
  • Add a method to read a string

In the case of platforms without a file system, or where interactive user input is not possible, the trait could have default implementations for these methods in order to not burden someone using libjinko.

• Add a Writer/Output trait

  • Add a method to write to an equivalent of stdout
  • Add a method to write to an equivalent of stderr

• Add default StdWriter and StdReader types which implement Writer and Reader using existing stdlib functions, such as fs::read_to_string, println!(), eprintln!()

• Remove reading/writing without the trait throughout the jinko library

  • In the Incl instruction, files are read and parsed through fs::read_to_string()
  • Errors are emitted using eprintln!()
  • The parser reads a file using fs::read_to_string
  • There might be other places where this is the case

Ideally, this should allow us to compile the jinko library in a no_std + alloc environment. We could think about developing a tiny no_std repl example to test this.

We should not bother about FFI at this point, even though this issue clearly targets platforms where libffi might not be available.

[Skallwar]

The ideal API for "libjinko" users (such as the interpreter) would be something like that:

- let mut ctx = Context::new();
+ let mut ctx = Context::<DefaultReader, WasmWriter>::new();

After some tests, having a generic context have a HUGE domino effect on the codebase. So I will go the Box<dyn Reader> route

[Skallwar]

First (draft) PR landed #569