Implement MCOPY (#18)

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

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

@@ -21,6 +21,22 @@ void * memcpy(void *dst, const void *_src, size_t len) {
     return dst;
 }
 
+void * memmove(void *dst, const void *src, size_t n) {
+	char *d = dst;
+	const char *s = src;
+
+	if (d==s) return d;
+	if ((uintptr_t)s-(uintptr_t)d-n <= -2*n) return memcpy(d, s, n);
+
+	if (d<s) {
+		for (; n; n--) *d++ = *s++;
+	} else {
+		while (n) n--, d[n] = s[n];
+	}
+
+	return dst;
+}
+
 void *  __sbrk(uint32_t size) {
     uint32_t address;
     __asm__ __volatile__(