Add gdb.Value.format_string ()

Add gdb.Value.format_string ()

The str () function, called on a gdb.Value instance, produces a string
representation similar to what can be achieved with the print command,
but it doesn't allow to specify additional formatting settings, for
instance disabling pretty printers.

This patch introduces a new format_string () method to gdb.Value which
allows specifying more formatting options, thus giving access to more
features provided by the internal C function common_val_print ().

gdb/ChangeLog:

2019-04-01  Marco Barisione  <mbarisione@undo.io>

	Add gdb.Value.format_string ().
	* python/py-value.c (copy_py_bool_obj):
	(valpy_format_string): Add gdb.Value.format_string ().
	* NEWS: Document the addition of gdb.Value.format_string ().

gdb/doc/ChangeLog:

2019-04-01  Marco Barisione  <mbarisione@undo.io>

	* python.texi (Values From Inferior): Document
	gdb.Value.format_string ().

gdb/testsuite/ChangeLog:

2019-04-01  Marco Barisione  <mbarisione@undo.io>

	Test gdb.Value.format_string ().
	* gdb.python/py-format-string.exp: New test.
	* gdb.python/py-format-string.c: New file.
	* gdb.python/py-format-string.py: New file.