Cut over to just using .cljc and Clojure reader conditionals

[arrdem]

With the introduction of reader conditionals, there isn't a particularly good reason to stay on the cljx ecosystem.

This changeset just cuts over from .cljx files to .cljc files, making use of reader conditionals where possible to maximize structural sharing between dialects.

Doesn't test cleanly on my machine due to the test case using textual rather than pattern structural equality on regexes.

running test task ["test"] ...

lein test frak-test

lein test :only frak-test/pattern-test

FAIL in (pattern-test) (frak_test.clj:59)
expected: (= "b(?:i[pt]|at)" (string-pattern ["bat" "bip" "bit"] nil))
  actual: (not (= "b(?:i[pt]|at)" "b(?:at|i[tp])"))

[noprompt]

@arrdem I want to apologize for letting this sit here for so long. 😭 I completely failed to see this! I really appreciate that you put this together. Merging!