Better broken mappings

Buildable source tarball: wfmash-v0.18.0.tar.gz

What's Changed

• It patches by @ekg in #264

This release fixes a bunch of small issues with previous updates to the mapping merging and splitting logic.

The main update should improve mapping coverage by correctly calculating the block length of the mapping based on the pre-split mapping. We also correctly organize cuts to be in regions without SVs.

Full Changelog: v0.18.0...v0.19.0