Refactor expressions

[bkeepers]

This is a fairly significant refactor of Expressions (#692).

1. Adds Constant (which quacks like an Expression) to represent explicit string, number, and boolean values. Note that there are still String, Number, and Boolean expressions, which will perform casting of values passed to them, but those expressions are no longer used by default to represent constant values when serialized:

   {
     "Equal" => [
       {"Property" => ["plan"]},
   -     {"String" => ["basic"]},
   +     "basic",
     ]
   }    ```

2. A new expression is now a simple object that responds to #call and receives any arguments instead of subclassing Expression. All evaluating of arguments is done internally in Expression before being passed to the #call method. If the context: key is part of the method signature, it will be passed in.

   - class Flipper::Expressions::Property < Expression
   -   def initialize(args)
   -     super Array(args)
   -   end
   - 
   -   def evaluate(context = {})
   -     key = evaluate_arg(0, context)
   -     context.dig(:properties, key.to_s)
   -     end
   -   end
   - end
   + class Flipper::Expressions::Property
   +   def self.call(key, context:)
   +     context.dig(:properties, key.to_s)
   +   end
   + end

This greatly simplifies the process of adding new expressions internally, and paves the way for making it easy for users to create expressions of their own.

[jnunemaker]

Looks great. I left a few minor things.

My only concern is the time stuff. I think it's too liberal right now for multi-language. We'll probably want to lock it down and only accept strings in a particular format or formats.

[jnunemaker]

This will assume local time, not utc, won't it? I stumbled on this stack overflow when googling.

[bkeepers]

Only if the string passed in excludes the timezone. In the schemas I'm working on, I explicitly defined Time to take a date-time, which JSON Schema defines as an ISO8601 formatted string. Here are the tests:

    Time
      valid
        ✓ {"Time":"2021-01-01T00:00:00Z"}
        ✓ {"Time":["2021-01-01T00:00:00Z"]}
        ✓ {"Time":"2021-01-01T00:00:00-05:00"}
        ✓ {"Time":["2021-01-01T00:00:00-05:00"]}
        ✓ {"Time":{"Property":"created_at"}}
        ✓ {"Time":[{"Property":"created_at"}]}
      invalid
        ✓ {"Time":"2021-01-01"}
        ✓ {"Time":"January 1, 2021 10:00"}
        ✓ {"Time":null}
        ✓ {"Time":false}
        ✓ {"Time":[{"Property":"created_at"},{"Property":"updated_at"}]}

[bkeepers]

I should add I'm planning to pull those schemas in and validate the input in Expression.build.