Synthetic benchmark on a few pending PRs combined.

[andersio]

Benchmark Source
#106 + #112 + #107 + #113

Seven scenarios

1. Send 10000 events to 1 observer of a plain signal.
2. Send 10000 events to 8 observers of a plain signal.
3. Send 10000 events to 1 observer of a produced signal from a 5-times-transformed producer.
4. Send 10000 events to 2 observer of a produced signal from a 5-times-transformed producer.
5. Start, send one event to and dispose a 5-times-transformed producer for 10000 times.
6. Start a 5-times-transformed producer for 10000 times.
7. Start and dispose a 5-times-transformed producer for 10000 times.

TL;DR

1. A non-trivial regression in sending events. (as a result of the interruption handling fix in Mitigate race conditions in the interruption handling. #112). Transformed producer would amplify the effect.
2. But if there is enough work done for each event (e.g. invoking eight empty observers), the regression would be outweighed.
3. Single-digit-percentage improvements in the SignalProducer starting & disposing performance.
4. When it comes to a common pattern for a scrolling collection views (rebinding reusable cells to different producers, represented by scenario 5), there is a minor net improvement.

Further optimization

1. Synchronise Signal.observers using read-copy-update. This removes a lock from the critical path of a Signal, at a cost of more expensive observer insertion.

// -Owholemodule
// `signal-state-patch` + `disposable-lockfree` + `lift-fix` + `minor-signal-refactor`
One Observer:    0.026 12%
Eight Observers: 0.042 8%
Producer:        0.326 3%
Producer 2 obv:  0.632 2%
Producer Mixed:  2.038 2%
Start Producer:  1.582 2%
Start & Dispose: 1.581 2%

// `master`
One Observer:    0.018 10%
Eight Observers: 0.053 5%
Producer:        0.223 3%
Producer 2 obv:  0.422 2%
Producer Mixed:  2.090 2%
Start Producer:  1.663 2%
Start & Dispose: 1.683 2%

// Speedup
One Observer:    0.692x ***
Eight Observers: 1.262x
Producer:        0.684x ***
Producer 2 obv:  0.668x ***
Producer Mixed:  1.025x
Start Producer:  1.051x
Start & Dispose: 1.065x