reorganize code around result formatting

Code is now organized by api version. So formatting has moved to
the api.v1 module. Instead of holding a separate ResultFormatter
object per result format, simply move the functions to the
formater collector and hand in the requested format as a parameter.
Thus reorganized, the api.v1 module can export three simple functions
for result formatting which in turn makes the code that uses
the formatters much simpler.

nominatim/api/result_formatting.py

@@ -0,0 +1,56 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2023 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Helper classes and functions for formating results into API responses.
+"""
+from typing import Type, TypeVar, Dict, List, Callable, Any
+from collections import defaultdict
+
+T = TypeVar('T') # pylint: disable=invalid-name
+FormatFunc = Callable[[T], str]
+
+
+class FormatDispatcher:
+    """ Helper class to conveniently create formatting functions in
+        a module using decorators.
+    """
+
+    def __init__(self) -> None:
+        self.format_functions: Dict[Type[Any], Dict[str, FormatFunc[Any]]] = defaultdict(dict)
+
+
+    def format_func(self, result_class: Type[T],
+                    fmt: str) -> Callable[[FormatFunc[T]], FormatFunc[T]]:
+        """ Decorator for a function that formats a given type of result into the
+            selected format.
+        """
+        def decorator(func: FormatFunc[T]) -> FormatFunc[T]:
+            self.format_functions[result_class][fmt] = func
+            return func
+
+        return decorator
+
+
+    def list_formats(self, result_type: Type[Any]) -> List[str]:
+        """ Return a list of formats supported by this formatter.
+        """
+        return list(self.format_functions[result_type].keys())
+
+
+    def supports_format(self, result_type: Type[Any], fmt: str) -> bool:
+        """ Check if the given format is supported by this formatter.
+        """
+        return fmt in self.format_functions[result_type]
+
+
+    def format_result(self, result: Any, fmt: str) -> str:
+        """ Convert the given result into a string using the given format.
+
+            The format is expected to be in the list returned by
+            `list_formats()`.
+        """
+        return self.format_functions[type(result)][fmt](result)