common: Make use of [[nodiscard]] where applicable

Now that clang-format makes [[nodiscard]] attributes format sensibly, we
can apply them to several functions within the common library to allow
the compiler to complain about any misuses of the functions.

src/common/hash.h

@@ -5,36 +5,11 @@
 #pragma once
 
 #include <cstddef>
-#include <cstring>
 #include <utility>
 #include <boost/functional/hash.hpp>
-#include "common/cityhash.h"
-#include "common/common_types.h"
 
 namespace Common {
 
-/**
- * Computes a 64-bit hash over the specified block of data
- * @param data Block of data to compute hash over
- * @param len Length of data (in bytes) to compute hash over
- * @returns 64-bit hash value that was computed over the data block
- */
-static inline u64 ComputeHash64(const void* data, std::size_t len) {
-    return CityHash64(static_cast<const char*>(data), len);
-}
-
-/**
- * Computes a 64-bit hash of a struct. In addition to being trivially copyable, it is also critical
- * that either the struct includes no padding, or that any padding is initialized to a known value
- * by memsetting the struct to 0 before filling it in.
- */
-template <typename T>
-static inline u64 ComputeStructHash64(const T& data) {
-    static_assert(std::is_trivially_copyable_v<T>,
-                  "Type passed to ComputeStructHash64 must be trivially copyable");
-    return ComputeHash64(&data, sizeof(data));
-}
-
 struct PairHash {
     template <class T1, class T2>
     std::size_t operator()(const std::pair<T1, T2>& pair) const noexcept {