Add MapMemory::unmap

[wdcui]

    // TODO: if the copy or protect step fails, the pages allocated by
    // map_zero are leaked because the MapMemory trait has no unmap
    // method, and no caller cleans up partially-mapped segments either.
    // Add an `unmap` method to MapMemory and clean up the reserved
    // region on failure in ElfParsedFile::load().