be13_api-related changes for bulk_extractor+lightgrep work

This pull request needs to be accepted in turn with the bulk_extractor pull request.

The change here is that I've removed static linkage from truncate_at() in feature_recorder_set.cpp, so it will be exposed, and I've put it's definition in bulk_extractor_i.h. Bulk_extractor itself has a duplicate of truncate_at() in another source file, which I have deleted.