Move segment types into a new registers::segmentation module

[phil-opp]

The segment types themselves and the related traits do not require the instructions feature, so I think that it makes sense to move them to the top-level registers module. The instructions::segmentation module still re-exports all the types, so this change should not break any code. I also moved the SegmentSelector type from the gdt module to the new module since it's more related to the segment registers than to the GDT.

This should fix cargo doc errors reported in #307 since the segment types are now available even without the instructions feature. Some special handling was necessary for the GsBase and KernelGsBase types: They now use a #[cfg_attr(feature = "instructions", doc = "...")] to append the link to the swap method only when it is available.

To ensure that we don't regress on this, I added cargo doc --no-default-features as a step in our CI script.

Fixes #307

[npmccallum]

@josephlr We are currently blocked from releasing the sgx crate due to this bug.

[josephlr]

Looks reasonable to me, I checked over everything and the docs (with and without default features) look reasonable. We could just remove the entire x86_64::instructions::segmentation module when we remove the deprecated free functions.