Remove dependency on guava

[dmuino]

Ideally servo shouldn't depend on guava. This would only leave slf4j as a runtime dependency which is quite OK.

[ryanrupp]

Provided the Guava classes are not part of the public API you could shade guava into servo-core, I'm not sure if you mind the bloat though (also not sure offhand what tool Gradle uses to do this, looks like fatJar or maybe this port of maven shade - https://github.com/johnrengelman/shadow)

[dmuino]

There are a couple of instances where we expose guava classes in the public API: for example DoubleGauge and MetricTransformObserver.

Shading is an option, but it's easy enough to remove the dependency (other than how to deal with the public API). I have a simple branch where I'm doing a quick pass at this: https://github.com/dmuino/servo/tree/guavaCleanup

[brharrington]

No plans to change the public api for servo at this time.