Steps within a job, with commands like restore_cache

Description

I'm not sure if this is a good idea or not, but looking at the new CircleCI 2.0 syntax, they let you declare steps within a single job, which isn't really new, but what they've done is given you control over steps outside our normal script operation. Like you can determine when you want to restore_cache. They used to be all about the magic (inference), but now have moved even farther towards explicitness than we have. I'm not sure if these steps are overriding otherwise default steps, or if nothing is default anymore, and if you left out the checkout step, for example, would it actually not checkout? I'm guessing the latter.

e.g. https://github.com/VividcodeIO/feature9/blob/master/circle.yml

version: 2
jobs:
  build:
    working_directory: ~/circleci-feature9
    docker:
      - image: maven:3.5.0-jdk-9

    steps:
      - checkout

      - restore_cache:
          key: circleci-feature9-{{ checksum "pom.xml" }}

      - run: mvn test

      - save_cache:
          paths:
            - ~/.m2
          key: circleci-feature9-{{ checksum "pom.xml" }}

      - store_test_results:
          path: target/surefire-reports

Proposal

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Overview

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