Extract Dataset and use api instead of state

[delner]

This pull requests makes two refactors to Evals and Datasets.

Issue: Dataset is internal to Eval

Right now, datasets are entirely internal to Eval and inaccessible for other purposes.

Idea: Extract it to a first-class API

dataset = Braintrust::Dataset.new(name: "my-dataset", project: "my-project")
dataset.each { |record| puts record[:input] }

Issue: Use of state for Eval.run

Right now, Eval uses state as an argument to run:

Braintrust::Eval.run(
  project: "my-project",
  dataset: "my-dataset",
  task: ->(input) { classify(input) },
  scorers: [...],
  state: state # <-- Uses Braintrust::State
)

This seems undesirable, given that State is meant to be an internal construct, rather than a public one... if we make changes to state management, it could cause a breaking change.

Idea: Encapsulate within api

Rather than expose state: as an argument, expose api: instead. This simplifies the logic, and substitutes an internal construct for a more meaningfully public one.

[clutchski]

I'm not sure about this one. Will comment tmw