Add $_as_string convenience function

Add $_as_string convenience function

This patch is a follow-up to "Add printf format specifier for printing
enumerator":

  https://sourceware.org/ml/gdb-patches/2016-02/msg00144.html

Instead of having a solution specific to the printf command, Pedro
suggested adding a general purpose function $_as_string() that would
cover this use case and more.

So, in order to print the textual label of an enum, one can use:

  (gdb) printf "Visiting node of type %s\n", $_as_string(node)
  Visiting node of type NODE_INTEGER

gdb/ChangeLog:

	* data-directory/Makefile.in (PYTHON_FILE_LIST): Install
	gdb/function/as_string.py.
	* python/lib/gdb/function/as_string.py: New file.
	* NEWS: Mention the new $_as_string function.

gdb/testsuite/ChangeLog:

	* gdb.python/py-as-string.exp: New file.
	* gdb.python/py-as-string.c: New file.

gdb/doc/ChangeLog:

	* gdb.texinfo (Convenience Functions): Document $_as_string.