Fix strSlice assertion

Signed-unsigned comparison giving bad results for the comparison.  Should probably
turn -Wsign-compare back on.

src/runtime/str.cpp

@@ -1652,9 +1652,9 @@ Box* _strSlice(BoxedString* self, i64 start, i64 stop, i64 step, i64 length) {
     assert(step != 0);
     if (step > 0) {
         assert(0 <= start);
-        assert(stop <= s.size());
+        assert(stop <= (i64)s.size());
     } else {
-        assert(start < s.size());
+        assert(start < (i64)s.size());
         assert(-1 <= stop);
     }
     assert(length >= 0);

test/tests/str_functions.py

@@ -181,3 +181,10 @@ def irgen_error():
 		fail = "test {0} {1} {2}".format(1, 2, 3)
 		print fail
 irgen_error()
+
+s = "hello"
+for i in xrange(-8, 8):
+    for j in xrange(-8, 8):
+        print i,j, repr(s[i:j])
+        for k in (-2, 1, 1, 2):
+            print i,j,k, repr(s[i:j:k]), repr(s[slice(i, j, k)])