Add bench of modulo op

benchmarks/bench_mod.nim

@@ -0,0 +1,33 @@
+import ../src/mpint, times
+
+
+# Warmup on normal int
+var start = cpuTime()
+block:
+  var foo = 123
+  for i in 0 ..< 10_000_000:
+    foo += i*i mod 456
+    foo = foo mod 789
+
+# Compiler shouldn't optimize away the results as cpuTime rely on sideeffects
+var stop = cpuTime()
+echo "Warmup: " & $(stop - start) & "s"
+
+####################################
+
+
+start = cpuTime()
+block:
+  var foo = 123.initMpUint(64)
+  for i in 0 ..< 10_000_000:
+    let i2 = i.initMpUint(64)
+    foo += i2 * i2 mod 456.initMpUint(64)
+    foo = foo mod 789.initMpUint(64)
+
+stop = cpuTime()
+echo "Library: " & $(stop - start) & "s"
+
+# On my i5-5257 broadwell with the flags:
+# nim c -d:release -d:mpint_test
+# Warmup: 0.040888s
+# Library: 5.838267s