*: add "noexcept" to many, many function prototypes

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

src/ReplayGainMode.hxx

@@ -36,7 +36,7 @@ enum class ReplayGainMode : uint8_t {
  */
 gcc_pure
 const char *
-ToString(ReplayGainMode mode);
+ToString(ReplayGainMode mode) noexcept;
 
 /**
  * Parse a string to a #ReplayGainMode.  Throws std::runtime_error on