Replace memory::external::mapped_region_t with unique_region, or rebase the former on the latter

[eyalroz]

The memory::external::mapped_region_t class is pretty similar to a memory::unique_region. Perhaps we can replace the former with the latter, or at least make the latter a child class of the former.

[eyalroz]

Completely clean replacement! Did not even have to correct a single command anywhere :-)