Suggest adding set-precision! API to accompany with-precision

Ticket #137 makes math-context</code> settable at the REPL. However, <code>math-context is not a public name in Clojure.

The related public function is with-precision</code> which works by pushing a new binding for <code>math-context</code>. This ticket suggests adding <code>set-precision!</code> to Clojure's public API. Its effect would be the same as <code>with-precision</code>, but accomplished by using <code>set!</code> on the current <code>math-context binding rather than by pushing a new binding.

Chouser suggests that we also add a doc string for math-context</code> noting that it is private and pointing the user to <code>with-precision</code> and <code>set-precision!. I agree.