modstruct: Improve compliance with python3

While checking whether we can enable -Wimplicit-fallthrough, I encountered
a diagnostic in mp_binary_set_val_array_from_int which led to discovering
the following bug:
```
>>> struct.pack("xb", 3)
b'\x03\x03'
```
That is, the next value (3) was used as the value of a padding byte, while
standard Python always fills "x" bytes with zeros.  I initially thought
this had to do with the unintentional fallthrough, but it doesn't.
Instead, this code would relate to an array.array with a typecode of
padding ('x'), which is ALSO not desktop Python compliant:
```
>>> array.array('x', (1, 2, 3))
array('x', [1, 0, 0])
```
Possibly this is dead code that used to be shared between struct-setting
and array-setting, but it no longer is.

I also discovered that the argument list length for struct.pack
and struct.pack_into were not checked, and that the length of binary data
passed to array.array was not checked to be a multiple of the element
size.

I have corrected all of these to conform more closely to standard Python
and revised some tests where necessary.  Some tests for micropython-specific
behavior that does not conform to standard Python and is not present
in CircuitPython was deleted outright.

1 files changed, 5 insertions, 4 deletions

diff --git a/py/binary.c b/py/binary.c
index cd0f1aa4d..b85edba62 100644
--- a/py/binary.c
+++ b/py/binary.c
@@ -126,7 +126,6 @@ mp_obj_t mp_binary_get_val_array(char typecode, void *p, mp_uint_t index) {
             break;
         case BYTEARRAY_TYPECODE:
         case 'B':
-        case 'x':   // value will be discarded
             val = ((unsigned char*)p)[index];
             break;
         case 'h':
@@ -330,7 +329,11 @@ void mp_binary_set_val(char struct_type, char val_type, mp_obj_t val_in, byte **
         }
     }
 
-    mp_binary_set_int(MIN((size_t)size, sizeof(val)), struct_type == '>', p, val);
+    if (val_type == 'x') {
+        memset(p, 0, 1);
+    } else {
+        mp_binary_set_int(MIN((size_t)size, sizeof(val)), struct_type == '>', p, val);
+    }
 }
 
 void mp_binary_set_val_array(char typecode, void *p, mp_uint_t index, mp_obj_t val_in) {
@@ -379,8 +382,6 @@ void mp_binary_set_val_array_from_int(char typecode, void *p, mp_uint_t index, m
         case 'B':
             ((unsigned char*)p)[index] = val;
             break;
-        case 'x':
-            ((unsigned char*)p)[index] = 0;
         case 'h':
             ((short*)p)[index] = val;
             break;