Compute outside-to-outside TLENs properly

[adamnovak]

Changelog Entry

To be copied to the draft changelog by merger:

• TLEN for read 1 starting inside read 2 will no longer be shorter than the union of the reads in vg surject.
• TLEN computation handles more CIGAR operations in case we decide to use them.

Description

Even if the reads overlap we need an outside-to-outside TLEN like the function claims to calculate.

This adds a bunch of unit tests for TLEN computation edge cases based on the SAM spec, and makes us pass them.