Add block['var'] accessor

Add block['var'] accessor

Currently we support iteration on blocks; this patch extends that to make
subscript access work as well.

gdb/ChangeLog:

2019-08-05  Christian Biesinger  <cbiesinger@google.com>

	* NEWS: Mention dictionary access on blocks.
	* python/py-block.c (blpy_getitem): New function.
	(block_object_as_mapping): New struct.
	(block_object_type): Use new struct for tp_as_mapping field.

gdb/doc/ChangeLog:

2019-08-05  Christian Biesinger  <cbiesinger@google.com>

	* python.texi (Blocks In Python): Document dictionary access on blocks.

gdb/testsuite/ChangeLog:

2019-08-05  Christian Biesinger  <cbiesinger@google.com>

	* gdb.python/py-block.exp: Test dictionary access on blocks.