py: Add casting to viper; add native mem stores to viper.

Viper can now do the following:

def store(p:ptr8, c:int):
    p[0] = c

This does a store of c to the memory pointed to by p using a machine
instructions inline in the code.

tests/micropython/viper_ptr8_store.py

@@ -0,0 +1,29 @@
+# test ptr8 type
+
+@micropython.viper
+def set(dest:ptr8, val:int):
+    dest[0] = val
+
+@micropython.viper
+def memset(dest:ptr8, val:int, n:int):
+    for i in range(n):
+        dest[i] = val
+
+@micropython.viper
+def memset2(dest_in, val:int):
+    dest = ptr8(dest_in)
+    n = int(len(dest_in))
+    for i in range(n):
+        dest[i] = val
+
+b = bytearray(4)
+print(b)
+
+set(b, 42)
+print(b)
+
+memset(b, 43, len(b))
+print(b)
+
+memset2(b, 44)
+print(b)

tests/micropython/viper_ptr8_store.py.exp

@@ -0,0 +1,4 @@
+bytearray(b'\x00\x00\x00\x00')
+bytearray(b'*\x00\x00\x00')
+bytearray(b'++++')
+bytearray(b',,,,')