Move pack/unpack to box_lua.m from lua/init.m

Preparation for addition of 't' format specifier in
pack/unpack: move these functions to box_lua.m.
Merge branch 'stable' of github.com:mailru/tarantool into stable
Merge branch 'stable'

Conflicts:
	src/box/box_lua.m
	src/lua/init.m
	test/box/lua.result