Place FmtLogMessage's definition in backend.cpp

I decided to overload LogMessage because I don't see a reason to come up with a new function name just for this, but if you guys want me to overload FmtLogMessage instead I'm fine with that.

src/common/logging/backend.h

@@ -14,6 +14,26 @@ namespace Log {
 
 class Filter;
 
+/**
+ * A log entry. Log entries are store in a structured format to permit more varied output
+ * formatting on different frontends, as well as facilitating filtering and aggregation.
+ */
+struct Entry {
+    std::chrono::microseconds timestamp;
+    Class log_class;
+    Level log_level;
+    std::string filename;
+    unsigned int line_num;
+    std::string function;
+    std::string message;
+
+    Entry() = default;
+    Entry(Entry&& o) = default;
+
+    Entry& operator=(Entry&& o) = default;
+    Entry& operator=(const Entry& o) = default;
+};
+
 /**
  * Returns the name of the passed log class as a C-string. Subclasses are separated by periods
  * instead of underscores as in the enumeration.
@@ -25,5 +45,9 @@ const char* GetLogClassName(Class log_class);
  */
 const char* GetLevelName(Level log_level);
 
+/// Creates a log entry by formatting the given source location, and message.
+Entry CreateEntry(Class log_class, Level log_level, const char* filename, unsigned int line_nr,
+                  const char* function, std::string message);
+
 void SetFilter(Filter* filter);
 } // namespace Log