Implements a barebones repl

Thought I'd take a stab at implementing a barebones REPL. I'm not sure if it fits the criteria from #41 (closed) for two reasons: 1) it's written in Rust, not Inko, and 2) it doesn't link to any external readline library, but it does add a dependency on the rust crate rustyline to handle readline-type functionality.

Closes #41 (closed)