Optional first() & last() procedure arguments for data_partition_t

With this commit, the data_partition_t type-bound procedures first() and last() no longer require an argument. Not passing an argument is treated equivalently to passing this_image() as the argument.

What's Changed

• feat(data_partition): make last/first arg optional by @rouson in #51

Full Changelog: 3.9.1...3.9.2