Add mg_crc32

src/util.c

@@ -273,6 +273,16 @@ int64_t mg_to64(struct mg_str str) {
   return result * neg;
 }
 
+uint32_t mg_crc32(uint32_t crc, const char *buf, size_t len) {
+  int i;
+  crc = ~crc;
+  while (len--) {
+    crc ^= *(unsigned char *) buf++;
+    for (i = 0; i < 8; i++) crc = crc & 1 ? (crc >> 1) ^ 0xedb88320 : crc >> 1;
+  }
+  return ~crc;
+}
+
 double mg_time(void) {
 #if MG_ARCH == MG_ARCH_WIN32
   SYSTEMTIME sysnow;