implement calldatacopy

Signed-off-by: xermicus <cyrill@parity.io>

crates/pallet-contracts-pvm-llapi/src/polkavm_guest.c

@@ -12,6 +12,15 @@ void * memset(void *b, int c, size_t len) {
     return b;
 }
 
+void * memcpy(void *dst, const void *_src, size_t len) {
+    uint8_t *dest = dst;
+    const uint8_t *src = _src;
+
+    while (len--) *dest++ = *src++;
+
+    return dst;
+}
+
 
 // Exports