Set up a benchmark of the runtime

We'll want to take some representative proc macro(s) to compile to wasm, and set up a benchmark that times how long it takes to expand some representative inputs.

This would be a necessary first step toward beginning to optimize the Watt runtime.

The issue names no files or tests; start by surveying the repository for the runtime and representative procedural-macro examples. Define a benchmark that compiles representative proc macros to WebAssembly and times expansion on representative inputs; done means repeatable timing data is available for future optimization.

Written by the indexing model from the issue text.