mmio: fix duplicate transactions in integral read/write methods

the integral write methods (e.g. write32, write16, write8) were causing
duplicate writes on the memory bus, apparently due to the underlying
implementation of packing / unpacking a memory mapping slice. this
commit modifies the integral read and write methods to dereference a
ctypes pointer of the appropriate width instead, which was tested and
shown to only yield one transaction on the bus. special thanks to
@paul-demo for raising this issue, investigating solutions, and testing.

resolves #50.

periphery/mmio.py

@@ -105,7 +105,7 @@ def read32(self, offset):
 
         offset = self._adjust_offset(offset)
         self._validate_offset(offset, 4)
-        return struct.unpack("=L", self.mapping[offset:offset + 4])[0]
+        return ctypes.c_uint32.from_buffer(self.mapping, offset).value
 
     def read16(self, offset):
         """Read 16-bits from the specified `offset` in bytes, relative to the
@@ -127,7 +127,7 @@ def read16(self, offset):
 
         offset = self._adjust_offset(offset)
         self._validate_offset(offset, 2)
-        return struct.unpack("=H", self.mapping[offset:offset + 2])[0]
+        return ctypes.c_uint16.from_buffer(self.mapping, offset).value
 
     def read8(self, offset):
         """Read 8-bits from the specified `offset` in bytes, relative to the
@@ -149,7 +149,7 @@ def read8(self, offset):
 
         offset = self._adjust_offset(offset)
         self._validate_offset(offset, 1)
-        return struct.unpack("B", self.mapping[offset:offset + 1])[0]
+        return ctypes.c_uint8.from_buffer(self.mapping, offset).value
 
     def read(self, offset, length):
         """Read a string of bytes from the specified `offset` in bytes,
@@ -196,7 +196,7 @@ def write32(self, offset, value):
 
         offset = self._adjust_offset(offset)
         self._validate_offset(offset, 4)
-        self.mapping[offset:offset + 4] = struct.pack("=L", value)
+        ctypes.c_uint32.from_buffer(self.mapping, offset).value = value
 
     def write16(self, offset, value):
         """Write 16-bits to the specified `offset` in bytes, relative to the
@@ -220,7 +220,7 @@ def write16(self, offset, value):
 
         offset = self._adjust_offset(offset)
         self._validate_offset(offset, 2)
-        self.mapping[offset:offset + 2] = struct.pack("=H", value)
+        ctypes.c_uint16.from_buffer(self.mapping, offset).value = value
 
     def write8(self, offset, value):
         """Write 8-bits to the specified `offset` in bytes, relative to the
@@ -244,7 +244,7 @@ def write8(self, offset, value):
 
         offset = self._adjust_offset(offset)
         self._validate_offset(offset, 1)
-        self.mapping[offset:offset + 1] = struct.pack("B", value)
+        ctypes.c_uint8.from_buffer(self.mapping, offset).value = value
 
     def write(self, offset, data):
         """Write a string of bytes to the specified `offset` in bytes, relative