Exports of non-public perf APIs were accidentally removed

[chrispcampbell]

During the refactoring for #471, I moved the perf module in the runtime package out of the model-runner directory to the top level, but forgot to add it to the top-level index.ts so that it remains exported. The perf module is not considered part of the public API yet but we have an internal tool that relies on it, so it should be restored as an export.