bpo-39320: Handle unpacking of **values in compiler (GH-18141)

* Add DICT_UPDATE and DICT_MERGE bytecodes. Use them for ** unpacking.

* Remove BUILD_MAP_UNPACK and BUILD_MAP_UNPACK_WITH_CALL, as they are now unused.

* Update magic number for ** unpacking opcodes.

* Update dis.rst to incorporate new bytecodes.

* Add blurb entry.

10 files changed