Refactor compression and XOR last model value

[skejserjensen]

This branch started as an endeavor to improve the compression of residual values by XORing the first residual value with the model's last value instead of storing the first residual value in full. However, doing so proved to be very difficult due to the less than ideal structure the compression crate had ended up with (a combination of functions and builders with unclear separation of responsibility) as functionality was added over time. Thus, I had to give up halfway and refactor the compression crate. The crate now consists of the modules list below and the compression process is performed by three, hopefully, intuative builders (ModelBuilder -> CompressedSegmentBuilder -> CompressedSegmentBatchBuilder) while compression.rs only operates as the driver that decides how to represent each part of the time series being compressed.

• lib.rs: re-export of the few functions and type (ErrorBound) intended for users of the crate.
• types.rs: types used throughout the crate.
• compression.rs: compression of time series as segments containing metadata and models.
• merge.rs: merging of segments containing metadata and models.
• models/*.rs: fitting of models to time series and computing aggregates from segments containing metadata and models.

So while this PR make significant changes to the compression crate it is mostly refactoring. I have tried to make the PR simpler to review by rewriting the commit history so the refactoring is done first. Thus, a diff containing only the commits made after the refactoring can be seen here.

[chrthomsen]

This was a challenge to read through :-) I did not find not any problems