Prework

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Description

Metdata like bytes gets corrupted when we store it because _targets/meta/meta is a text file. We need a new size field to help keep track of when files change is such a way that avoids unnecessary re-hashing on large files.

WARNING: this change will probably invalidate workflows completed with earlier versions of targets.

Despite the interruption, I think we need this because it will improve the accuracy and efficiency with which targets tracks files. In addition, this package has no CRAN release yet, so this is still a permissible time for breaking changes.