modernize library benchmarks

[taylorswift]

we need to modernize swift-png’s benchmarks. this will involve:

1. moving the benchmarks into a nested SPM project, with a separate Package.swift manifest. this frees up the benchmarks to adopt additional dependencies, and is the model @karwa ’s swift-url uses, and has already been adopted by swift-json.

2. updating the benchmarking code to use modern stdlib features, like ContinuousClock.

3. configuring benchmarks to run as part of the project’s CI pipeline, the way swift-json currently does it.

4. integrate the library’s performance reporting system into the CI pipeline, and automate this process. (right now it is a completely manual pre-submit procedure!)

5. add standards to the CI pipeline, so that major performance regressions will fail the status checks.

6. backport items # 4 and # 5 to swift-json so that it can also benefit from these improvements.

7. copy this setup to swift-jpeg, so it can also benefit from this infrastructure, as part of tayloraswift/jpeg#3 .