Improve referencing data fields for StepFunctions tasks, in preparation
of callback task implementaion.

Get rid of `JsonPath`, and in its place we have 2 new classes:

- `DataField`, for fields that come from the user payload
  (`$.My.Field`). Settle on the term "data" since that's the
  term used in most of StepFunctions' docs.
- `ContextField`, for fields that come from the service-defined
  task "context" (like `$$.Execution.StartTime`, and in particular
  `$$.Task.Token`).

These classes have been moved from the `-tasks` module to the
`aws-stepfunctions` module, where it seems to make more sense for them
to live.

Add support for SQS and SNS tasks to publish an arbitrary JSON
structure that can reference fields from context and execution data.

Remove `NumberValue` since we can now encode Tokens in regular number
values.

BREAKING CHANGES:

- `JsonPath.stringFromPath` (and others) are now called `DataField.fromStringAt()`.
  The `DataField` class now lives in the main stepfunctions module.
- `SendToQueue` property `messageBody` => `message`.
- `PublishToTopic` property `messageObject` used to take a pre-JSONified
  object, now directly accepts a JSON object.
- Instead of passing `NumberValue`s to StepFunctions tasks, pass regular
  numbers.