Added hash functions as a tool to generate serialization ids or just

hash objects during compile time.

README.md

@@ -450,12 +450,40 @@ Note that the serialization ids of types in the variant must match in length, or
 compilation error will issue.
 
 You may also use any sequence of bytes instead of a readable string, as well as an integer
-or any literal type.
+or any literal type, here is an example of how to use a hash of a string as a serialization
+id:
+```cpp
+using namespace zpp::bits::literals;
+
+// Inside:
+using serialize_id = zpp::bits::id<"v1::person"_sha1>; // Sha1
+using serialize_id = zpp::bits::id<"v1::person"_sha256>; // Sha256
+
+// Outside:
+auto serialize_id(const person &) -> zpp::bits::id<"v1::person"_sha1>; // Sha1
+auto serialize_id(const person &) -> zpp::bits::id<"v1::person"_sha256>; // Sha256
+```
+
+You can also serialize just the first bytes of the hash, like so:
+```cpp
+// First 4 bytes of hash:
+using serialize_id = zpp::bits::id<"v1::person"_sha256, 4>; // First 4 bytes of sha256
+```
 
 The type is then converted to bytes at compile time using (... wait for it) `zpp::bits::out`
 at compile time, so as long as your literal type is serializable according to the above,
 you can use it as a serialization id.
 
+* If you want to serialize the variant without an id, or if you know that a variant is going to
+have a particular ID upon deserialize, you may do it using `zpp::bits::known_id` to wrap your variant:
+```cpp
+std::variant<v1::person, v2::person> v;
+
+ // Id assumed to be v2::person, and is not serialized / deserialized.
+out(zpp::bits::known_id<"v2::person"_sha256, 4>(v));
+in(zpp::bits::known_id<"v2::person"_sha256, 4>(v));
+```
+
 * As part of the library implementation it was required to implement some reflection types, for
 counting members and visiting members, and the library exposes these to the user:
 ```cpp

test/src/test_hash.cpp

@@ -0,0 +1,20 @@
+#include "test.h"
+
+namespace test_hash
+{
+using namespace zpp::bits::literals;
+using namespace std::literals;
+
+static_assert(""_sha1 ==
+              "da39a3ee5e6b4b0d3255bfef95601890afd80709"_unhexlify);
+static_assert(
+    ""_sha256 ==
+    "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"_unhexlify);
+
+static_assert("The quick brown fox jumps over the lazy dog"_sha1 ==
+              "2fd4e1c67a2d28fced849ee1bb76e7391b93eb12"_unhexlify);
+
+static_assert(
+    "The quick brown fox jumps over the lazy dog"_sha256 ==
+    "d7a8fbb307d7809469ca9abcb0082e4f8d5651e46d3cdb762d02d0bf37c9e592"_unhexlify);
+} // namespace test_hash