Implement field numbers in format strings

Implement field numbers in format strings

A field number explicitly specifies the argument to be formatted.
This is especially important for potential localization work, since
grammars of various languages dictate different word orders.

* src/editfns.c (Fformat): Update documentation.
(styled_format): Implement field numbers.

* doc/lispref/strings.texi (Formatting Strings): Document field numbers.

* lisp/emacs-lisp/bytecomp.el (byte-compile-format-warn): Adapt.

* test/src/editfns-tests.el (format-with-field): New unit test.