Make dependency on file location in Block explicit

[eivindjahren]

A small refactoring to make dependency of the Block::write_data() byte offset explicit.

The Block::write_data() function depended on being called with the data_stream being at the location
it should write, which was unlike any of the other write_* functions. The write_header() function would
guarantee that this was the case. This ment the order of operations in write was very much significant although
it may not look like it.

The write_* functions follow a general pattern which is not uncommon. They fseek() in the stream to where they should write and afterwords stream is at the end of where we wrote. Another common pattern is to not enforce the location, and simply write wherever the stream is currently pointing, but the current design makes that infeasible for all of the functions, yet write_data() followed that convention. Instead of having this surprising inconsistency, the write_header() function now supplies the data offset which is passed to write_data(), making the dependency explicit.

All the write_* functions now have the following behavior

1. move to some predetermined location in the stream
2. Write your thing there
3. After finishing the stream location is after where it wrote.

Points 2. and 3. is the same behavior as fwrite. Point 1. is a reasonable convention.

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.

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[dafeda]

Nice 👍