Introduce a Timer Snapshot Mechanism

[CryZe]

This introduces the ability to create snapshots of the timer. At the moment of snapshot creation the current time of the timer is evaluated. The snapshot then represents the timer with the current time frozen at that specific time. This allows layout state evaluation and other calculations to work with a consistent timer that is entirely frozen, allowing for more consistent results. However almost more importantly, this reduces the amount of times we need to take time stamps from the underlying system, improving the performance a bit.

In the web the performance improvement is even more significant. There the layout state calculation improved from 8.5us to 5us.

[wooferzfg]

Overall, this makes sense to me.

Something that I'm curious about - have we thought about how Snapshots will fit in with Time Sources (#90) once those are implemented? I think that the Time Sources would allow us to also freeze the current_time calls that we make from within Timer, such as when splitting:

livesplit-core/src/timing/timer/mod.rs

Line 317 in 706361a

let current_time = self.current_time();

[CryZe]

The Synchronization Protocol and the Time Sources are mostly concerned about how you modify / control the timer. We need some low level mechanism that does indeed affect

livesplit-core/src/timing/timer/mod.rs

Line 317 in 706361a

let current_time = self.current_time();

For perceiving the timer however the snapshot mechanism is what we want. I don't think this clashes with the design of manipulating the timer.