string-table: Introduce DECLARE_STRING_TABLE_LOOKUP() and friends

Let's introduce these for the following two reasons:
- Using them makes sure the parameter names match between declaration
  and definition (if both use the corresponding macros).
- They make sure developers can't forget the _const_ and _pure_
  attributes for the declarations.

This commit also includes some include sorting fixes

src/basic/time-util.h

@@ -232,5 +232,4 @@ int usleep_safe(usec_t usec);
 
 int time_change_fd(void);
 
-const char* timestamp_style_to_string(TimestampStyle t) _const_;
-TimestampStyle timestamp_style_from_string(const char *s) _pure_;
+DECLARE_STRING_TABLE_LOOKUP(timestamp_style, TimestampStyle);