Add swap_run_mid_attempt for mid-run route swapping

[shookietea]

This assumes the frontend goes past simply using Game - Category distinction and instead has an option for routes. This could look like Majora's Mask - Glitchless Any% - 2 Bottle No Aliens. An alternative route in this case could be failing to complete the Goron Race, and must leave to get Epona and complete the Gormon Brothers' Race. In that example, Goron Race is moved later, as you must get Epona before a specific amount of IGT passes. Most of the time, segments can be set up so that it doesn't matter, but you know what they say. More is more

The main use case for this function is as follows:

• Part way through a run, you make a mistake which causes a route divergence
• Normally, the runner will skip segments until they get back on route, or just skip segments to the final segment to complete timing as they are uncertain of the remaining segments' accuracy
• With this, a route change is possible so long as the route attempting to swap to has the same previous segments
• This allows the run to continue on the swapped to route, which allows the runner to split accurately

This seemed to be the cleanest prerequisite to adding mid-run route swapping for a timer frontend I have in development. Making this change in livesplit-core allows for clean moving of segment information. Let me know if you'd like to chat about this or have a better solution using the available public methods