Add an overload to xxhash to allow it to take a string constant (#189)

People were using the xxhash function with string constants like
`xxhash32("Hello");` and it was compiling just fine, and then returning
the hash of the first letter.

This adds an overload which makes xxhash32("Hello"); use a template with
an `N` array in order to hash that correctly.

Author: TrentHouliston

src/util/serialise/xxhash.hpp

@@ -44,8 +44,13 @@ namespace util {
         uint32_t xxhash32(const char* input, const size_t& length, const uint32_t& seed = 0);
 
         template <typename T>
-        uint32_t xxhash32(const T* input, const uint32_t& seed = 0) {
-            return xxhash32(reinterpret_cast<const char*>(input), sizeof(T), seed);
+        uint32_t xxhash32(const T& input, const uint32_t& seed = 0) {
+            return xxhash32(reinterpret_cast<const char*>(&input), sizeof(T), seed);
+        }
+        template <uint32_t S = 0, size_t N>
+        // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays,modernize-avoid-c-arrays)
+        uint32_t xxhash32(const char (&input)[N]) {
+            return xxhash32(input, N - 1, S);
         }
 
         /**
@@ -62,8 +67,13 @@ namespace util {
         uint64_t xxhash64(const char* input, const size_t& length, const uint64_t& seed = 0);
 
         template <typename T>
-        uint64_t xxhash64(const T* input, const uint64_t& seed = 0) {
-            return xxhash64(reinterpret_cast<const char*>(input), sizeof(T), seed);
+        uint64_t xxhash64(const T& input, const uint64_t& seed = 0) {
+            return xxhash64(reinterpret_cast<const char*>(&input), sizeof(T), seed);
+        }
+        template <uint64_t S = 0, size_t N>
+        // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays,modernize-avoid-c-arrays)
+        uint64_t xxhash64(const char (&input)[N]) {
+            return xxhash64(input, N - 1, S);
         }
 
     }  // namespace serialise