faster crc32 by doing slicing-by-4

pkg2zip_crc32.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include "pkg2zip_utils.h"
+
+typedef struct {
+    MSVC_ALIGN(16) uint32_t crc GCC_ALIGN(16);
+} crc32_ctx;
+
+void crc32_init(crc32_ctx* ctx);
+void crc32_update(crc32_ctx* ctx, const void* buffer, size_t size);
+uint32_t crc32_done(crc32_ctx* ctx);