Factor out m_seq_get_fast_slice_indexes() fucntions as sequence helper.

Takes slice object and sequence length and computes subsequence indexes
for case of slice step=1.

py/obj.h

@@ -395,3 +395,4 @@ typedef struct _mp_obj_classmethod_t {
 
 // sequence helpers
 void mp_seq_multiply(const void *items, uint item_sz, uint len, uint times, void *dest);
+bool m_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, machine_uint_t *begin, machine_uint_t *end);

py/objstr.c

@@ -115,26 +115,8 @@ mp_obj_t str_binary_op(int op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
                 }
 #if MICROPY_ENABLE_SLICE
             } else if (MP_OBJ_IS_TYPE(rhs_in, &slice_type)) {
-                machine_int_t start, stop, step;
-                mp_obj_slice_get(rhs_in, &start, &stop, &step);
-                assert(step == 1);
-                if (start < 0) {
-                    start = lhs_len + start;
-                    if (start < 0) {
-                        start = 0;
-                    }
-                } else if (start > lhs_len) {
-                    start = lhs_len;
-                }
-                if (stop <= 0) {
-                    stop = lhs_len + stop;
-                    // CPython returns empty string in such case
-                    if (stop < 0) {
-                        stop = start;
-                    }
-                } else if (stop > lhs_len) {
-                    stop = lhs_len;
-                }
+                machine_uint_t start, stop;
+                assert(m_seq_get_fast_slice_indexes(lhs_len, rhs_in, &start, &stop));
                 return mp_obj_new_str(lhs_data + start, stop - start, false);
 #endif
             } else {

py/sequence.c

@@ -23,3 +23,33 @@ void mp_seq_multiply(const void *items, uint item_sz, uint len, uint times, void
         dest = (char*)dest + copy_sz;
     }
 }
+
+bool m_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, machine_uint_t *begin, machine_uint_t *end) {
+    machine_int_t start, stop, step;
+    mp_obj_slice_get(slice, &start, &stop, &step);
+    if (step != 1) {
+        return false;
+    }
+
+    // Unlike subscription, out-of-bounds slice indexes are never error
+    if (start < 0) {
+        start = len + start;
+        if (start < 0) {
+            start = 0;
+        }
+    } else if (start > len) {
+        start = len;
+    }
+    if (stop <= 0) {
+        stop = len + stop;
+        // CPython returns empty sequence in such case
+        if (stop < 0) {
+            stop = start;
+        }
+    } else if (stop > len) {
+        stop = len;
+    }
+    *begin = start;
+    *end = stop;
+    return true;
+}