py: Convert [u]int to mp_[u]int_t where appropriate.

Addressing issue #50.
diff --git a/py/repl.c b/py/repl.c
index 2c3dfbd..66a67e3 100644
--- a/py/repl.c
+++ b/py/repl.c
@@ -31,7 +31,7 @@
 #if MICROPY_HELPER_REPL
 
 bool str_startswith_word(const char *str, const char *head) {
-    int i;
+    mp_uint_t i;
     for (i = 0; str[i] && head[i]; i++) {
         if (str[i] != head[i]) {
             return false;