improve correctness and speed of duplicate filter. (#1144)

improve correctness and speed of duplicate filter. (#1144)

* improve correctness and speed of duplicate filter.

The duplicate filter could errouneously delete points that were
not duplicates if the crc's happened to match.

waypt_del(Waypoint*) is inefficent as it requires a search of the
list to find the matching waypoint.  Support waypt_del with iterators.

* retire util_crc.cc

* improve duplicate to linear complexity

* polish new list creation.

* Remove final remnants of 'exported'

* Revert "Remove final remnants of 'exported'"

This reverts commit 6996e4d7dd4b1812f922edd2c5bd9a3d1bc6fada.

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Co-authored-by: Robert Lipe <robertlipe@gmail.com>