performance optimisation: precompute last zone change index of shortcut

[jannikmi]

Each Timezone Polygon corresponds to a zone id.
Each shortcut (h3 hexagon) points to a list of polygon ids sorted after their zone ids.
Currently during each lookup it is being computed where the list of sorted zone ids change for the last time (last_zone_change_idx).
to speed up the lookups, this value can be precomputed and stored as part of the binary file compilation in scripts/file_converter.py

[jannikmi]

relates to: #294 (higher priority)

[jannikmi]

benchmarks show: in practice the overall throughput is dominated by the point-in-polygon work.