Closes #354
Closes #311

Accomplishes a number of things: cleaner test suite running, a more parallel CI matrix (with one job per Clojure version), and a cleaner experience for developing Eastwood (namely: being able to fire up a repl, run tests from the repl, etc without having Lein internals possibly interfering).

Importantly, it makes :eval-in dependent on an env var (sadly it cannot be set depending on a Lein profile). This makes dogfooding Eastwood possible, while also not coupling the choice of Clojure version to Lein's (otherwise either test-all or dogfooding would break).

Commit summary

• CircleCI: use Lein 2.9.5
• Make dogfooding pass
  • the dogfooding had been broken for at least some months - which the cleaned-up project.clj setup revealed
• Make the test suite pass without the implicit dev profile
  • And particularly, without the leiningen-core dependency which makes it plausible that the test suite could be passing for fragile reasons.
• Extract :warn-on-reflection profile
  • This way, one can opt to launch a repl process without *warn-on-reflection*, which can be noisy, in face of arbitrary deps outside our control.
• Move leiningen.eastwood to its own source path
  • This way one can use tools.namespace (refresh) without needing the leiningen-core dependency to be present (which tends to be a bad practice in non-plugins)
• project.clj :eastwood: remove :only-modified true
  • Because the Eastwood implementation can always change, caching linting results can easily cache past implementations' output, leading to false negatives.
• Make the matrix fully parallel
  • Now the matrix is 2-dimensional (JVM + clojure version), which results in a faster feedback loop and cleaner output (vs. different test suites being rendered as a single CI job)