Add diff support between files to heaptrack_{print,gui}.

Add diff support between files to heaptrack_{print,gui}.

Both executables get a new `--diff <file>` argument. The file
that is passed gets parsed in addition to the primary data file.
The allocation data therein will then be subtracted from the data
found in the primary data file.

The algorithm is somewhat complicated, because we have to ignore
instruction pointer addresses, as these will vary between runs
due to address space randomization. Paired with (partial) lack of
debug symbols, we end up with non-trivial situation to map allocations
from one run to that of another.

Essentially, we need to find the correct allocation that shares the
same backtrace while ignoring instruction pointer addresses.