Add const and constexpr specifiers to Key union

Making the member functions of Key `const' explicitly flags that they
will not change the union. This will allow to use Key in const contexts.

Adding the `constexpr' specifier to the function makes it possible to
rely on the results at compile time. This puts some kind of restrictions
on the function, especially when using C++11 and not a newer standard,
but these restrictions were already fulfilled, so this seems to be safe.

src/key_defs.h

@@ -26,41 +26,41 @@ union Key {
   };
   uint16_t raw;
 
-  inline bool operator==(uint16_t rhs) {
+  constexpr inline bool operator==(uint16_t rhs) const {
     return this->raw == rhs;
   }
-  inline bool operator==(const Key rhs) {
+  constexpr inline bool operator==(const Key rhs) const {
     return this->raw == rhs.raw;
   }
   inline Key& operator=(uint16_t raw) {
     this->raw = raw;
     return *this;
   }
-  inline bool operator!=(const Key& rhs) {
+  constexpr inline bool operator!=(const Key& rhs) const {
     return !(*this == rhs);
   }
-  inline bool operator>=(uint16_t raw) {
+  constexpr inline bool operator>=(uint16_t raw) const {
     return this->raw >= raw;
   }
-  inline bool operator<=(uint16_t raw) {
+  constexpr inline bool operator<=(uint16_t raw) const {
     return this->raw <= raw;
   }
-  inline bool operator>(uint16_t raw) {
+  constexpr inline bool operator>(uint16_t raw) const {
     return this->raw > raw;
   }
-  inline bool operator<(uint16_t raw) {
+  constexpr inline bool operator<(uint16_t raw) const {
     return this->raw < raw;
   }
-  inline bool operator>=(const Key& other) {
+  constexpr inline bool operator>=(const Key& other) const {
     return this->raw >= other.raw;
   }
-  inline bool operator<=(const Key& other) {
+  constexpr inline bool operator<=(const Key& other) const {
     return this->raw <= other.raw;
   }
-  inline bool operator>(const Key& other) {
+  constexpr inline bool operator>(const Key& other) const {
     return this->raw > other.raw;
   }
-  inline bool operator<(const Key& other) {
+  constexpr inline bool operator<(const Key& other) const {
     return this->raw < other.raw;
   }
 };