py/obj.h: Fix mp_seq_replace_slice_no_grow to use memmove not memcpy.

Because the argument arrays may overlap, as show by the new tests in this
commit.

Also remove the debugging comments for these macros, add a new comment
about overlapping regions, and separate the macros by blank lines to make
them easier to read.

Fixes issue #6244.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/memoryview_slice_assign.py

@@ -61,3 +61,27 @@ try:
     memoryview(array.array('i'))[0:2] = b'1234'
 except ValueError:
     print('ValueError')
+
+# test shift left of bytearray
+b = bytearray(range(10))
+mv = memoryview(b)
+mv[1:] = mv[:-1]
+print(b)
+
+# test shift right of bytearray
+b = bytearray(range(10))
+mv = memoryview(b)
+mv[:-1] = mv[1:]
+print(b)
+
+# test shift left of array
+a = array.array('I', range(10))
+mv = memoryview(a)
+mv[1:] = mv[:-1]
+print(a)
+
+# test shift right of array
+a = array.array('I', range(10))
+mv = memoryview(a)
+mv[:-1] = mv[1:]
+print(a)