Reuse sandbox containers across build commands

[syphar]

Sorry, this is a bigger one. Happy to walk you through the ideas / changes, happy to discuss them.

Doing some tests I realised that only docker create and docker rm sometimes takes 5s, 10s, or 40s. Also very often it's <100ms).

Docs.rs is running multiple commands:

1. coverage
2. docs
3. json

for each target.

This leads me to the assumption that this could be a major reason why there is such a big difference between local host-builds and docs.rs container builds for the same crate. In some examples I checked the numbers roughly fit, #125 will give us more data when added to docs.rs & deployed.

Between these commands, we don't need a new container, we can just run more commands in the same container.

This refactors rustwide to make this possible. I think the new design is a better one, so I made this a breaking change.

The idea is:

• SandboxBuilder just creates the Container
• on the Container we can then run one or more commands.

The previous ProcessStatistics had to go too, and are SandboxStatistics now (= per-container, not per-command/call). Our source in docker is exactly that, and the API should reflect it. This is fine for the optimized docs.rs usage.

testing

• i extended the test-suite here
• in WIP: build speed improvements docs.rs#3325 I ran the docs.rs build-tests with this branch.

[GuillaumeGomez]

Just nits, the rest looks good to me, thanks!

[GuillaumeGomez]

Thanks!