rename run_list -> span
array data structure wip
Introduce persistent-data/data-structures/
persistent-data/cache.h hadn't been checked in.
Introduce a mode enum to block_io<>
Add a CREATE mode to the block_io class.
[block_t] All block managers are created in create_bm.
[block] use lseek to size a new block file rather than fallocate.

Which sadly returned operation not supported for some reason.
[block_t] remove the test.data file before each test.
[Build] Dependencies weren't being built or included for the unit tests.
[unit-tests] Update block_manager constructors to use the block_io<>::mode param.