Consider conversions for expression literals/constants.

[daemontus]

We could probably add From implementation for the Expression type that would convert bool, String, &str, etc. into a Constant or Literal expression. Using generics, we could then make methods like binary_or work with any type that converts into an Expression. This would simplify building expressions in code, but could lead to confusion if we overdo it. I'm not quite sure what route to take here, but I'm leaving this here as a suggestion.