Expression evaluator and repl

[ahdinosaur]

• add rimu-env lib crate with Environment (previously Context)
• change rimu-value Value::Number to be rust_decimal
• add rimu-eval lib crate with evaluate function
  • add basic errors: EvalError
• add rimu-repl bin crate to play with parsing and evaluating expressions
• change operators in rimu-expr to more match Rust, e.g:
  • and -> &&
  • or -> ||
  • = -> ==
  • not -> !
  • mod -> %