Utility class for COW size calculation

The Linux kernel's dm-snapshot relies on a COW device to keep trace of
all the modified sectors.
The COW device has a precise structure, which allows to compute its
space requirement in advance, as a function of:
- sector size;
- chunk size;
- list of modifications applied to the snapshot device.

Create a class that implements the COW device space occupancy given this
information.

Bug: 140835698
Test: libsnapshot_test
Change-Id: I50e3815741ee689d14fc3611532ff9d3b3e0e879
Signed-off-by: Alessio Balsini <balsini@google.com>