Add ability to simply, fully verify file checksum

For accepting E57 data in automated processing, it would be awesome if libE57Format could offer a simple way to verify (the checsums of) an entire file with a simple function that can be called ahead of time, vs noticing `E57_ERROR_BAD_CHECKSUM` during (potentially more time consuming) stream processing.

If we wanted to add a `.checkAll()` function, how should it best be designed to generically traverse all data while still being as fast as possible?

Are there some slower approaches that would do the trick until then?

Finally, after a file has been verified this way, what would be the best way to disable checksumming during subsequent stream processing?

The issue names no files, tests, or entry points. Start by locating the existing checksum and stream-processing APIs, then resolve the proposed full-file verification design and how later checksumming would be disabled. Done means the API, traversal behavior, performance trade-offs, and post-verification behavior are clearly specified.

Written by the indexing model from the issue text.