Performance improvement in xmr_utils namespace

[kno3comma14]

If the team still using the function ->public-key from the namespace bips.xmr-utils then I purpose to change the following code:

(defn exp-mod
  "Calculates the result of a modular exponentiation of a given base `b`, exponent `e`, and modulus `m`."
  [b e m]
  (if (= e 0)
    (biginteger 1)
    (let [t (atom (.mod  (.pow (exp-mod (biginteger b) (.divide (biginteger e) (biginteger 2)) m) 2) m))]
      (when (not= (.and (biginteger e) (biginteger 1)) 0)
        (reset! t (.mod (.multiply @t (biginteger b)) m)))
      @t)))

with

(defn exp-mod
  [b e m]
  (.modPow b e m))

The internal implementation from BigInteger reduces the time by 3 seconds approximately. You can check the change here.