Resolves amperity#52.

This is a first pass for this feature so design input is requested.

Initial decisions:

- Use `use-fixtures` from `clojure.test` for fixture functions that
  apply to all of the tests in a namespace. Note that this does not
  support `:once` fixtures as of yet because the runner does not run all
  tests in a namespace as a group that can be `:once`d. This will
  require a change to `greenlight.runner`.
- Add `::test/each` as an option to the `deftest` attribute map. This
  needs to be a collection of fixture functions. This could not quite be
  expressed via the `use-fixtures` method because it would unilaterally
  apply a fixture function to all steps in all tests in a namespace.
  This could be an additional feature as well.

Open questions:
- Are functions expressive enough, or should this introduce a new
  namespace that allows for composable definitions of fixtures in the
  way that steps are composable? Something like `deffixture`, a
  greenlight-specific `use-fixture` for top-level, and then fixture
  configuration at the test/step level.
- Should context be passed into the fixture functions, and should it be
  modifiable by the fixture functions? e.g., should downstream steps see
  the changed context and should the test output contain that changed
  context. This gets a bit sticky and pushes on the `deffixture` idea,
  in that greenlight tries to be explicit about its data dependencies
  and outputs, but with fixtures that can take/update context, declaring
  data dependencies gets a bit hairier. I can imagine that `deffixture`
  would _also_ declare inputs and outputs, which could then get merged
  into the step inputs/outputs. Depends on how complex this needs to be.