Add an ID to Symbol in order to make hashing faster

Summary:
Before: 157M cycles, 252M instructions, 42.93 ms
After: 136M cycles, 165M instructions, 38.34 ms

Test Plan:
perf stat -r ./bench_parser heaptrack.david.18594.gz
It still shows "365 entries, 2896 allocations"
as before (unlike whenever I introduced bugs, so this is a useful check)

Reviewers: mwolff

Reviewed By: mwolff

Differential Revision: https://phabricator.kde.org/D24099